Can I use UltraDeck over a deck that has been previously coated with a oil based decking oil?

Yes, if the timber has weathered and lost all water repellency (test by seeing if water still beads on the surface). The timber should then be cleaned with UltraPrep Timber Cleaner and UltraDeck applied. If the timber is water repellent, the deck can be left to weather further (until it no longer water beads) or it can be sanded back.