How can I protect a shed floor from ground moisture?
I've built several such sheds in a climate that's quite moist and humid in summer. My strategy is to frame the floor with treated lumber, as you've done, then overlay either poly sheeting or house wrap, stapling it to temporarily hold position. You'll need to add some lumber around the perimeter to carry the floor along the walls, so do that first.
For flooring I've used untreated SPF tongue-and-groove plywood, which I give two coats of quality urethane after installation. This is mostly to protect against moisture draining from snow equipment and lawn hoses. I would not use OSB since it's more susceptible to moisture swelling and delamination in wet scenarios. OSB would be fine for the walls.
The first shed lasted 15 years with no serious degradation of the floor. The second one is about 3 years old and doing well. If you wanted to go the extra mile, sealing all sides of the flooring boards before installation would probably do some good in the second and third decades of the shed's life.