I've never been to IOP, but maybe you should ask why your psychiatrist and therapist think you need it. If you're fine right now, then maybe it was a misunderstanding.

If IOP conflicts with your schedule and you end up needing to go, you'll have to obviously request different shifts, but if you really don't think you need to attend such a program, then I would avoid it since you're a new employee. (Many bosses and other employees don't understand mental illness, and unfortunately, it's easy to fire the new guy and just replace him with someone else looking for a job. The employer can make up any BS excuse to fire you, like "omg Marcus stole a pen!! who knows what else he'll steal! we have to fire him just to be safe!") Hope that makes sense. (And yes, it's unfortunate how this world treats people with mental illness.)