My employer has a few sites that I work between. At first I only permanenrly worked 18 hrs/wk but now I am permanently working at another site with an extra 20 hrs so I work 38 hrs every week. What is CA law to determine if an employee is full or part-time? Also since it's a different office I get 2 different checks for each one. I work 10 hr days and get OT for the 2 hrs but if I work over 40 hours per week I don't get OT for those extra hours since it's not at the same location is this fair? The company does pay for over 8 hrs and over 40 hrs for OT.

California law is silent on the matter of defining full-time vs. part-time. I assume you're asking because of benefits, such as vacation or insurance? For vacation, the company policy would control; for insurance, the plan rules would control. Talk to your manager or corporate HR.

However, all hours at all sites must be considered when calculating overtime, since the establishments have common ownership; this is a federal requirement which California cannot make go away.
http://www.dol.gov/dol/allcfr/ESA/Title_29/Part_791/29CFR791.2.htm