Sorry to restart an old one, but plywood is best. At least a half inch if not 5/8 or 3/4. Those lifts are not light at all. When I worked tree service we used 3/4 for our bucket truck and never had issues with sinking even on the softest ground. OSB would work, but it's not the best choice IMO. OSB will not bend over ground imperfections and differnces like plywood will. Ground isn't always even, and if you're pulling into the yard from a road or driveway there may be as many as a few inches difference from the edge of the surfaces. OSB will likely break right along the line where as plywood will just crack a little bit. Some of the 3/4 sheets we had lasted us for 2-3 years plus.