Oracle has solid multi user support. It's data consistency routines are bullet proof and it has a wide range of user security management. However, Oracle is like a mechanical tool. You can give any wan**r a spanner and he'll make a mess of your car. As with other databases out there, Oracle is no different.
Read the concepts guide for a fuller description. I won't give you the link... if you can't find it yourself then you shouldn't be playing with any database
Please don't email me directly with questions. I've probably just got home from the pub and cannot guarantee the sanity of my answers. In fact, I can't believe I actually made it home.