Ostello says out near the tarmac, he suddenly saw a man come out of nowhere and jump inside the aircraft.
"He closed the door and I saw all the lights start coming on one by one. And that's when I realized I got to go do something with this guy. I got to get him out of this airplane before he destroys it," Ostello said.
Ostello says the unidentified man was just one button away from successfully turning the plane on.
Acting quickly, Ostello tells us he was able to remove the man from the plane.
After that, he says the man stood there for a second before taking off down the runway towards a hanger full of dozens of other planes.
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"I was thinking, oh no. I wanted him to run the other way and he just started running towards the airplane and I just thought, oh no," said Ostello.
Once he made it to the hanger, the man tried to steal a second plane.
This one owned by Ostello's boss, Robert Nicolas.
The man was stopped once again by Ostello - who had been following behind.
Ostello called authorities at the airport but not before the man ran away.
"He was looking up at the clouds. He was sniffing and grunting. He just kind of stood there for a second and then he just took off running," said Ostello.
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Airport officials tell ABC7 News they believe the suspect got onto the grounds here by entering in through a hole in a chain link fence, which has been undergoing renovations for the past couple of weeks.
"This was a momentary issue. The fence will be fully upgraded in the next two weeks so that it won't be an issue. We also do regular patrols of the perimeter and the property," said Jon Stout, the Sonoma County Airport Manager.
Nicolas says he doesn't blame the airport for what happened.
He does, however, credit Ostello for taking steps to avoid a potential disaster.
"The guy had shoved the throttle and the fuel forward, and he had the master on. If he would have hit the starter he probably would have caused about $300,000 worth of damage to the engine," said Robert Nicholas.
The Sonoma County Sherriff's Office says they're working to find and identify the man and are asking for the public's help.
As for Ostello, he's just happy no one was hurt and that he'll still be able to take the planes back into the sky.
"I just knew that I had to do something and that's what I did. I feel like I did the best thing I could possibly do," he said.