Well, you'll miss a whale of a good story if you don't finish the sequence! But yes, you can. As for Over Sea, Under Stone, I actually recommend that people skip that one when they start out. It's written for slightly younger children and doesn't really have the depth of the rest of the series. It's okay to go back and read later. If I'd started with it I don't know if I'd read the rest.
The people from OSUS do show up in the third book and meet Will. Then the last two tie up the whole story and bring it all to a conclusion.
However, if you just want to read Dark is Rising and not finish the rest, you can. Just depends on how much you enjoy or don't enjoy Dark.