I was in the float pool and worked at sites in Yukon, Bethany, Edmond, Chandler, NW Expressway, May Ave, and a couple of others. I really enjoyed most of them except Chandler (since I live in Tuttle that is a long way to drive). They did pay for mileage and the hourly rate is higher since you don't have guaranteed hours or any benefits. It depended on the time of year. At the holidays and when people want to take off for vacation I worked a lot. You are on call, there were days someone would call in sick, I would get a call at 7am and decide at that time if I was going to work or not.
A lot will depend on your schedule for school, they will work around it.
I really did enjoy Mercy. They are a good place to work.
Good luck with school!