An Ohio man who wrote online that he supported mass shootings and attacks at Planned Parenthood sites was busted at a home with a veritable arsenal inside, including over a dozen rifles, pistols and roughly 10,000 rounds of ammo, federal prosecutors said.

Justin Olsen, an 18-year-old who lives with his father in Boardman, was arrested Wednesday as he walked from the home to his car.

He had been under investigation since February, but a prosecutor in the town of 35,000 people southeast of Cleveland said he needed to act due to the recent mass shootings in Dayton, Ohio, that killed nine people, and in El Paso, Texas, that killed 22, Cleveland.com reports.

Authorities later seized a total of 15 rifles from the residence, including some assault-style models, as well as 10 semi-automatic pistols and roughly 10,000 rounds of ammunition. A large gun vault and a slew of camouflage clothing and backpacks were also found inside, according to an FBI affidavit.

Federal prosecutors allege that Olsen, using the handle “ArmyOfChrist,” voiced his support for mass shootings and coordinated attacks at Planned Parenthood locations on iFunny, a mobile app where users can share videos, memes and other content, the affidavit states.

In June, while talking about cult leader David Koresh and his group of nearly 80 Branch Davidians who were killed during a raid in 1993 that lasted 51 days, Olsen made his violent aspirations clear, according to an affidavit.

“In conclusion, shoot every federal agent in sight,” Olsen told fellow users.

Olsen also suggested that app users “stock up” on guns that federal authorities may one day ban, and expressed his support for Oklahoma City bomber Timothy McVeigh, saying that the domestic terrorist, who was executed in 2001, was proof that “armed resistance is a viable method of political change,” according to the affidavit.

Olsen was charged Monday with threatening to assault a federal law enforcement officer. He’s expected to return to court Friday for preliminary and detention hearings, Cleveland.com reports.

A message seeking comment from his attorney was not immediately returned Tuesday, but Olsen told FBI officials that the posts he made on iFunny from his cellphone were “only a joke” and downplayed his alleged call to shoot federal agents, saying that was a “hyperbolic conclusion based on the results of the Waco siege … where the ATF slaughtered families,” court documents show.

Olsen also wrote that he had been accepted into a Reserve Officers’ Training Corps program in Alabama and planned to move to Austin after being accepted at the University of Texas, according to a Boardman police report.

“Urging people to shoot federal agents is never acceptable,” US Attorney Justin Herdman said in a statement obtained by The Post. “This defendant has access to an arsenal and we take his threats very seriously.”