How good of a vapor barrier is OSB in a dirt crawlspace?
Moisture barriers are not a water tight seal kind of thing.

If you cover 75% of the dirt then you should reduce the mustiness by about that much.

Anything will probably be an improvement and if is not good enough you could paint the OSB floor to add some moisture barrier. However, this could possibly reduce the life of the OSB since it can't breath as well.

Good luck!