In Orange County , Ca. Can police officers do a probation search if there's no P.O. Assigned because it's informal probation?

Yes. The search clause imposed as part of the sentence reads that they can be searched any time day not night with or without a warrsnt, probable cause or reasonable suspicion by any peace or probation officer. That includes their person, property, residence ...

If he agreed to a search term as part of probation and he lives there, then yeah, they could legally search the house. They're only supposed to search common areas and areas under his control.