Physics and PAT aren't the only differences. The approach differs on the two tests. The questions on the OAT are a bit more terminology-based. The questions can partially be reduced to learning a series of definitions and key concepts. More specifically, the information are asked in much the same way as it's presented in the classroom. If you did well in the corresponding classes, then the OAT'll be quite easy. The DAT is also partially factual, in the sense that it covers the same material. But it's increasingly being applied to new situations you rarely see in class. The test writers try to present the question in a new and different way, so that's a small hurdle on the DAT.
There are official copies of both the DAT and OAT out there. You can verify everything I've said for yourself.