He Unlawfully Entered a Motor Vehicle (a Class A Misdemeanor)
Suppose that someone else recorded him saying, “I’m going to go steal that car over there. Oh, yeah, this is my big payday. Wait! What’s that? Somebody left a baby in here? It’s got to be a hundred ten inside this car! You can’t even see ’em in these new backwards baby seats. Call 9-1-1, I’ll get it out! Where’s the coolest place we can get to?”
That definitely proves he met the elements of the crime of Unlawful Entry into a Motor Vechicle (ORS 164.272).
A person commits the crime of unlawful entry into a motor vehicle if the person enters a motor vehicle, or any part of a motor vehicle, with the intent to commit a crime.
He Did not Commit Theft
Other answers state the definition of theft in Oregon (ORS 164.015) incorrectly. In order to commit theft, he would need to have
Take[n], appropriate[d], obtain[ed] or withhold[ed] such property from an owner thereof;
or one of several other things that don’t come close to applying here, such as extortion. Common-law definitions of theft do not pertain to the state of Oregon.
He Did not Commit Robbery
He would only have violated ORS 164, Oregon’s robbery or carjacking statute, if he had used or threatened to use force on another person.
He Probably Did not Commit Unauthorized Use of a Vehicle
The Oregon Court of Appeals suggested, in Oregon v. Phillips, 315 Or App 178 (2021), based on the legislative history of ORS 164.135 (Unauthorized Use of a vehicle), that to commit this crime, “a person would need to ‘exercise control,’ or otherwise start or somehow ‘use’ the vehicle.” (This was not a holding in the case, however.)
He Might not be Prosecuted
In the original scenario, his intent when he entered the car would be much more difficult to prove. It also seems extremely unlikely that the owner of the car would want to press any charges against the man who saved their baby’s life. (Or want everybody to find out that they nearly killed their own child.)
Even if the county prosecutor suspects that John was up to no good, they might not want to bring a case where there was no harm done (other than, possibly, damage to the lock), the victim doesn’t want publicity, the case comes down to a subjective judgment of intent, and the defendant is so sympathetic.