I think it's technically Oh-Wahz'. Although, I've often heard it Oh-Wahs'. It is NOT pronounced like Oh-Ase' or Oh-As'-eh

It is as they say it there:

O, ah, see

That is the way most cab drivers say it.

rod8239 is right - sometimes I hear an R at the end from a native german (or transplanted non-native resident).

It really doesn't matter - they know where you are going and you can just smile and say, 'guten tag'.