It completely depends on the setting. I have only worked in skilled nursing facilities, and in LA I was making $45/hr, and in San Diego, I'm making $48/hour. Do the math, and it's about $90k & 96k, respectively. So, in a word, yes it is possible to earn 80k+ in southern CA as a new grad. I only have a bit over one year's experience :) In terms of leveling off, from what I have heard, you typically don't break the $100-110k range unless you are particularly scrappy and go into home health or move out of treating entirely and into management, but I certainly cannot speak from experience. Typically, part time per diem or PRN (called in as needed) OTs make more than full time employments, as they don't receive benefits. From what I know, most OT positions pay by the hour, so you could expect a $4-5 increase from full time to part time/per diem rates to make up for the lack of benefits (PTO, health insurance, 401(k) match, etc.). Also, if you are working as an independent contractor, you're typically paid one rate for each hour treatment, and a significantly higher rate for each evaluation, as evaluations are reimbursed by insurance at a much higher rate.
Hope this helps and isn't too confusing :)