Master of Orion is turn-based game in which players alternate actions and decisions with computer-controlled opponents using a point-and-click interface as well as keyboard shortcuts to control the management of colonies, technology, ship construction, diplomacy, and combat. The game begins with a single colonized homeworld, one colony ship and two scout ships that can be used to explore nearby stars.

Victory is gained either by eliminating all opponents or by winning a vote on peaceful unification.

One planet is Orion, "throne world of the Ancients." Orion is the most valuable research site in the galaxy, draws support for its owner in the High Council, and hosts several technological advancements including the unresearchable "death ray." Orion also hosts a great robotic warship, the Guardian, that must be destroyed before the planet can be claimed.

Planetary population generates production, especially when assisted by factories. There is a limit on the number of factories a unit of population can operate, but players can increase this by researching and building upgrades. Lockable sliders are used to allocate a colony's output between ship construction, planetary defenses, factory construction, ecology or research. Within each of these industry sectors, there is a fixed sequence of activities to which resources are allocated. For example, defense effort is used to upgrade missile bases, then to build or upgrade planetary shields, and finally to build additional missile bases. Military and spy maintenance is deducted in proportion from every colony's production.

A planet's output can also be transferred to the treasury at a loss. The treasury can also be increased by scrapping ships or missile bases or by gifts from other empires. Money can be given as a gift, or used to boost a planet's production up to double the normal level.

The software generates a map randomly at the start of each game; the player's only influence over the map generator is the ability to choose the size of the galaxy and the number and difficulty of AI opponents. Star systems have at most one colonizable planet and a few have none. Planets vary in the following ways:

Mineral wealth dramatically influences a colony's industrial productivity when building or upgrading factories, building or upgrading defenses and building ships; it has no impact on the productivity of research nor of ecological improvements such as pollution control or terraforming.

Habitability influences population growth rates: fertile planets increase growth rates by 50% and Gaia planets by 100%, while hostile planets halve them. There are seven normal and six hostile planet types; the various hostile types require increasingly advanced technology to colonize, which extends the exploration and colonization phases of MoO for much longer than in most 4X games. Hostile planets are the most likely to be rich or ultra-rich in minerals. All planets can be upgraded to Gaia class with the appropriate technologies.

Size, which determines the planet's initial population capacity. This can be more than doubled by various kinds of terraforming.

Artifact worlds contain relics of a now-vanished advanced civilization. These usually provide a free technology advance to the first empire that discovers the planet, and always double the research productivity of a colony there, except that on Orion the research productivity of a colony is quadrupled.

Most techs are incremental improvements, but each field has unique techs.

The designers regard technology as the most important contribution to a player's success. The game's tech tree has six independent fields, and an empire can research one advancement in each simultaneously.

Computers: spaceship systems that improve combat effectiveness; factory controls that increase the number of factories each colonist can operate; scanners that monitor the movements of other empires' ships and eventually can even "explore" planets remotely; and a weapon that can destroy other ships' computer systems. Computer technology advances also improve the effectiveness of spies in both offensive and defensive operations.

Construction: reductions in the cost of building and upgrading factories; reductions in pollution; improved armor; and self-repair systems for ships.

Force fields: protective shields for ships, planets and ground troops; devices that make it harder to hit the players' ships; and some special weapons.

Planetology: reductions in the cost of pollution control; colonization of hostile planets; terraforming, which increases the maximum population of a planet; the ability to increase populations more efficiently; biological weapons and defenses against such weapons.

Propulsion: increases in the range and speed of starships; some special weapons and combat systems. Range increases are particularly important in early colonization.

Weapons for use by ships, missile bases and ground troops.

If a ship uses a component from a particular technology area, further advances in that area reduce the cost and size of the component; this effect is called "miniaturization". When one has researched all of the technologies in an area of the tech tree, further research can discover "advanced technologies" in that area, which do not provide specific new capabilities but increase the miniaturization of ship components.

Each advancement has a minimum point cost that gives a 1% chance of discovery per turn. The chance increases with further investment, reaching 100% at three times the minimum. Each research project returns "interest" on resources invested in it, using a formula that produces the greatest return if the project accounts for 15% of the total research budget.

Master of Orion provides a wide range of diplomatic negotiations: gifts of money or technology; one-time technology trades; trade pacts that boost industrial output; non-aggression and alliance treaties. Players can also threaten each other, declare war and arrange cease-fires Each AI player remembers others' actions, both positive and negative, and will be unwilling to form alliances with a player who has broken previous treaties with it.

Hostile actions do not automatically cause war. Clashes are even expected at the opening of the game, when all sides are sending probes out into the unknown. On the other extreme, a ground assault must be knowingly targeted at an inhabited planet, and is a massive provocation.

The ship design screen showing the selection of hull sizes and components.

Only six ship designs can be used at a time; beyond that, a previous slot must be emptied and all ships of that class scrapped before a new class can be designed. Ships cannot be upgraded or refitted with new technology,

There are four hull sizes; smaller sizes are harder to hit while larger ships can survive more damage and hold more components. There are eight types of components, each with different effects:

Battle computers increase the chance of a beam weapon hitting and damaging a target

Shields reduce the damage done by opponents' weapons

Electronic countermeasures reduce the risk of being hit by missiles

Armor determines the amount of damage a ship of a given hull size can withstand before being destroyed

Engines supply power to systems, and determine the speed of interstellar travel and a ship's maximum maneuverability during combat

Combat maneuverability determines how fast a ship can move during battle and how hard it is to hit; maximum maneuverability is determined by the engine type used

Weapons: missiles, beams, bombs and biological weapons (the last reduce a colony's population without damaging buildings)

Special systems which have varying effects: improve a ship's range or maneuverability; improve weapon accuracy or range; provide defensive, offensive, repair or sensing advantages; a few "special" weapons, some of which affect whole stacks of enemy ships; colony bases are also considered special systems

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Space combat screen. The numbers beside the ship icons show how many ships are in each stack.

Ships can travel to any star system within their range and combat always occurs in orbit over a planet - it is impossible to intercept enemy ships in deep space. All ships of the same class form a single stack, moving and firing as a unit. Players can control space combat manually or ask the software to resolve combat automatically. Battles are almost always decided by numbers and technology rather than by clever tactics. An attacker can bomb a planet during combat and, if it wins the space combat, can bomb more intensively immediately afterwards.

Colonies can be bombed out from space, or taken in ground invasions. Invasion is expensive: there are no special soldier units. Colonial population itself is sent to fight, exterminate the existing inhabitants, and form a new planetary population. However, it has several awards: the production capacity of any remaining factories, plundering of technologies if enough factories survived the attack, and control over a new system that extends the range of the invader's ships.

Ground invasions can be conducted through enemy defenses. Present enemy ships or missile bases will fire on the approaching transports, possibly destroying some or all of them. The invasion itself is fully automatic. Results depend on numbers, technology and (if one of the races involved is Bulrathi) racial ground combat bonus.

Master of Orion has 10 playable races, each with a specialty. For instance, the Humans have advantages in trade and diplomacy; the Bulrathi are the best at ground combat; the Silicoids ignore pollution and can colonize even the most hostile planets, but have slow population growth. Each race is predisposed to like or dislike some of the other races , and is advantaged or disadvantaged in different research fields.

Under AI control, each race has a ruler personality and an objective, such as Xenophobic Expansionist or Pacifistic Technologist. These traits guide their politics and economic management; for example militarists maintain large fleets and prioritize technologies which have military benefits, while ecologists put a lot of effort into pollution control and terraforming. Traits vary from game to game. Each race has most probable traits and avoids their opposites. Races may occasionally revolt and change traits.

Master of Orion will sometimes produce random events which can be harmful or advantageous, such as discovery of ancient ships and technology, changes to planetary conditions that alter the planet's population or mineral richness, diplomatic blunders/assassination attempts, changes to research, industry or treasury production, planetary rebellion, space piracy and either viruses or a supernova event (both of which require research from the affected planet to overcome). Random events can be disabled by means of a cheat code.