I know many creationists claim that there is no such thing as an Oort cloud, but just because we haven't observed it yet, doesn't mean the effects of it's existence are not visible, and like jcl pointed out, you can derive the comets route and origin, so why exclude the Oort cloud?

Astrology without (some) assumptions, then there would hardly be anything left I guess. Take for example the birth of new stars, we would never ever be able to witness a full life cycle of a star. It just takes too long to witness, so we assume other stars are in a certain phase of their life cycle and that's how we figured out how the life cycle probably goes. I think there are enough stars in different phases to be able to put the puzzle together, like a frame by frame movie.
For most assumptions there are motivations and reasons for scientists believing they are right. If an assumption turns out to be invalid, well then we just have to adapt or change the theory, not every false assumption will completely falsify a theory most of the time anyways. In science there is no truth in absolutes, it's knowledge and thus time related. If you are going to attack every unproven thing or every assumption science makes, then you might as well start with the creation theory itself, where's the proof for that, that's an assumption to isn't it? It's even an assumption that requires faith, because of the lack of any indirect evidence. Just my two cents.

Cheers