The OLP humbucker is wired as two sets of single coils, one set of coils goes to one volume control and the other set to the other volume control. It's basically a Jazz bass setup with the two pickups next to one another.
By the sound of it what you have done is added a Jazz pickup to the original and wired it to the front set of coils? If so you've now still got a Jazz bass, but with a bit more distance between the pickups, hence the phase cancelling.
One way to fix it is to wire the original pickup as a humbucker, possibly with a series/parallel switch or push/pull pot, and then add a pre-amp to give you blend, volume and some EQ.
Thread here http://basschat.co.uk/topic/14009-pimp-mah-olp/page__p__147925__hl__pimp%20olp__fromsearch__1#entry147925 has someone putting an Artec SE-2A into one.