If I just received my medical marijuana card but am on probation, is it OK for me to possess and use it?
No, it is not most likely ok. Even though medical marijuana may be legal, marijuana is otherwise still an illegal drug (for example: it is still a restricted substance under federal law). Probation may put restrictions on people which limit them in ways they would not be limited if not on probation. For example, someone on probation can be legally prohibited from associating with felons, even though someone not on probation may associate with them. It would be legal for your probation to state you may not use or possess illegal drugs (and this is a common restriction); if so, since marijuana is, as stated, still considered an illegal drug, possessing or using it would be a violation of your probation.