I know it's late, but for the record ....
The reason for your problem is that you have multiple unique indexes/constraints on the table. Presumably one is the oracle primary key, and why that one isn't automatically picked I have no idea.
The only way I have found to get it to pick the right one is to temporarily drop the indexes you don't want to use as the primary key, link the tables in Access, then recreate the other indexes. Problematic on a production db I know.
The reason RiverGuy always gets the option to choose the key is probably because he doesn't have any unique indexes defined on his table (tut tut ;-) ). He might also be linking everything through Oracle views.
Better late than never,
Nick.