Tables without a primary key (has_and_belongs_to_many association tables for example) do not have auto-generated sequences. On inserts into these tables the adapter tries to retrieve the next sequence value which causes an exception to be thrown because the sequence doesn't exist.
Patch here:
http://github.com/bbrowning/activerecord-jdbc-adapter/commit/88b2a0fcfe441ef3f64849d9c485aad6d78c9438
I haven't been able to get the oracle test suite running yet but once I do I'll add another patch with a test for this issue.