While on vacation in Oregon someone gets a DUI. He lives in
California.
In what state will the trial be?
Oregon. State law crimes are always tried in the jurisdiction where they are committed. A crime is committed either where the acts causing the crime took place or where the harm caused by the crime is directed. In the classic example, if you shoot and murder someone in Oregon from within California, either state (or both states) can prosecute you for murder. If you have a DUI in Oregon that doesn't involve crossing the state line, California can't try you for that crime.
If he's convicted, will he have to move to Oregon to serve probation
or would he automatically serve probation in California since he lives
there?
A standard condition of probation is to not leave the state without court permission, but the court imposing probation can authorize you to leave the state subject to other restrictions devised by the court (e.g. forfeiture of one's passport until the probation sentence is completed so that extradition is available if a felony is committed while on probation).
There is also a process in place by which states can cooperate to have their probation officers supervise a convicted defendant's probation order from one state in another state (usually with the requirement that the sentencing state either pay for that service, have the defendant pay for that service, or that there be reciprocity such that California will only agree supervise people placed on probation in Oregon if Oregon will agree to supervise people placed on probation in California).
It is also possible that even though probation is an available sentencing option, that for an out of state defendant, a jail sentence and/or a larger than usual fine will be imposed in lieu of a probation sentence, to avoid these complications.
Of course, unless you let your lawyer (or if you are representing yourself, the court) know what you would like to arrange and take the proper legal steps to do so (which realistically requires the skills of an experienced criminal lawyer), the court won't allow you to do so.
