Fourth in a series

An offensive line version of musical chairs begins in earnest Monday when unrestricted free agents can begin receiving parameters of offers from prospective employers.

That means that left tackle Donald Penn, a two-year starter and the blind side protector for Derek Carr, will soon have a good idea of his worth and whether the Raiders are in the mix to retain his services.

Players can’t actually be signed to contracts until Wednesday. (Players who have been released by their teams can sign immediately).

It’s worth remembering how Penn came to the Raiders two seasons ago. The Raiders, rather than re-up with Jared Veldheer before free agency, allowed him to hit the open market and instead made their big pitch to Rodger Saffold of the St. Louis Rams.

Veldheer quickly came to an agreement with the Arizona Cardinals and Saffold with the Raiders. Except Saffold failed his Oakland physical with a torn labrum, with owner Mark Davis strongly suggesting to general manager Reggie McKenzie to take a pass.

It was an embarrassing sequence, although in the long-run it didn’t matter much. Penn, coming off a poor season with Tampa Bay, became available and signed with the Raiders to replace Veldheer. And according to Profooballfocus, Penn even out-performed Veldheer.

So this time, do the Raiders again let their own player walk because they’ve got their eye on someone else? And is that someone Kalechi Osemele, a powerful 26-year-old blocker from the Baltimore Ravens?

The Raiders have some work to do up front. Over the last eight games of the season, lead rusher Latavius Murray averaged 3.3 yards per carry and Carr absorbed 23 of his 31 sacks, clear indications of getting whipped up front.

A look at some possibilities when free agents can be signed on March 9 (teams can open negotiations Monday), as well as players under contracts and their own free agents:

UNRESTRICTED FREE AGENTS

Kelechi Osemele, Baltimore: Opinion is split on whether Osemele at 6-5, 330 pounds, is a guard or tackle. He played the last four games of the season at left tackle for Baltimore with mixed results. But Osemele is a road-grader, only 26, and the biggest, most powerful presence on the market. McKenzie and Del Rio aren’t into finesse.

Russell Okung, Seattle: Once considered a top talent, the 6-5, 310-pound Okung has missed 13 games due to injury over the past three seasons and has yet to play a full 16 games in his 16-year career.

Mitchell Schwartz, Cleveland: A good enough pass blocker to have held Von Miller to one pressure in 2015, Schwartz is 6-5, 310 and plays primarily at right tackle. At age 26, hasn’t missed a game in five seasons after being a second-round draft pick out of Cal.

Kelvin Beachum, Pittsburgh: Depending on how Beachum checks out medically, he could be in for a huge payday. Had his season cut short by an ACL injury last year but is still only 27. Another player who can play both guard and tackle. Lacks ideal size at 6-3, 303.

Brandon Brooks, Houston: Brooks is 26 and has the requisite size at 6-4, 335 pounds and is the kind of physical drive blocker who would be a fit for the system. A three-year starter who goes the school in the offseason with designs on being an investment banker when his career is over.

Richie Incognito, Buffalo: The 6-3, 319-pound guard played all 16 games for Buffalo last season, rebuilding his image somewhat after the bullying issue in Miami. Strictly a guard, and McKenzie has been big on bringing in players who they are sure will be good teammates.

Jermon Bushrod, Chicago: A mainstay tackle with the Saints, where he was on a Super Bowl champion and played in the Pro Bowl as recently as 2012, the Bears signed Bushrod to a five-year, $35 million deal in 2013 with $17 million guaranteed but released him this offseason at age 31.

Andre Smith, Cincinnati: A right tackle at 6-4, 335 who has missed nine games over the past two years and generally believed to be on the decline. A former first-round draft pick out of Alabama.

Evan Mathis, Denver: After being released by Philadelphia, Mathis, 34, signed a one-year deal with Denver and got a Super Bowl ring. A capable player at 6-5, 301 pounds but better in zone scheme _ think along the lines of Cooper Carlisle.

Stefan Wisniewski, Jacksonville: As a Raiders rookie, he was a starting guard. Has been a center ever since and he wouldn’t be a center with the Raiders unless he backed up Rodney Hudson.

Joe Barksdale, San Diego: A 6-5, 326-pound tackle who was a Raiders third-round pick in 2011 who was phased out under Dennis Allen when they went to a zone scheme running attack. Could depth at both tackle spots.

PLAYERS UNDER CONTRACT

Rodney Hudson: The Raiders first big score in free agency a year ago will be the center of attention on the offensive line for the next several years.

Gabe Jackson: The most powerful run blocker on the line, Jackson, if all goes according to play, will be the left guard alongside Hudson for the foreseeable future.

Austin Howard: Struggled at guard in 2014, an idea from former line coach and interim coach Tony Sparano. Back at right tackle under Mike Tice, Howard performed well until a late-season knee injury.

Menelik Watson: The Raiders tried their best to make Watson their right tackle, but a torn Achilles’ in training camp put him on injured reserve. Tice still loves Watson’s athleticism and potential, but needs to stay healthy.

Jon Feliciano: Got in six games with three starts at the end of the season as a run-up to an important offseason and training camp entering Year 2.

Mitch Bell: At 6-3, 345 pounds, Bell was a reserve/future signing so the Raiders could get a look in the off-season program.

UNRESTRICTED FREE AGENTS

Donald Penn: A solid locker room leader and most important, a solid blindside protector for Carr the past two seasons. What is also true is the Raiders haven’t run well to the left side over the past two seasons.

J’Marcus Webb: A tackle by trade, Webb was moved to right guard and was a surprising 16-game starter. Did not grade out well, however, according to Profootballfocus and may be more suited to being a utility lineman capable of playing both guard and tackle.

Tony Bergstrom: The first-ever draft pick by McKenzie as a general manager, one year after being inactive for 16 straight games, earned good reviews from the coaching staff subbing for an injured Hudson three times at center.

Khalif Barnes: An 11-year veteran, including seven with the Raiders, Barnes made one start last season as coaches began to look to younger players. Value is as a swing guard and tackle.

RESTRICTED FREE AGENTS

Matt McCants: Reportedly signed a low-round tender, tying him to the Raiders unless another team believes he is worth a sixth-round draft pick.

EXCLUSIVE RIGHTS FREE AGENTS

None