Is conflict of interest a legal reason for immediate termination?

Yes, sadly, you can be terminated for nearly any reason. California is an "At Will" employment state. That means the employer can simply let you go for no reason at all. Of course, the employer cannot terminated based on national origin, religion, gender, race and those 'protected' areas. But for a reason like 'conflict of interest', that employer can decide they do not want an employee who has another job doing something they feel might overlap with their requirements. That said, that employer might be found to have to pay Unemployment Insurance, so go apply with the EDD for UI.

In Ohio, unless you have an employment contract that restricts termination, you are an "employee at will" and can be terminated for any reason or no reason, so long as it is not an illegal reason.From your question, I can't see how it would make any sense to terminate you, or even really be any conflict of interest, but nor do I see any legal prohibition against termination.Unless you make too much money from your other employment, you are probably eligible for unemployment based on being terminated without just cause, and you should probably consider pursuing an unemployment claim.