The Constitution applies to federal property as well.
However, state laws do not apply on Indian reservations. Oklahoma could ban abortion entirely and any one of the reservations in the state could open a clinic and there isn't a darned thing the feds or the state can do. Like casino gambling the tribes may see this as a money maker and take advantage of it.
One would hope that Native American tribes have more respect for life than to put money before life. One would hope.
Imagine an abortion shop among casino gift shops.