I am confused. I am reading through about MVCC in Oracle. I figured MVCC meant no locks. But, I just read elsewhere that UPDATEs do automatic securing, no matter the isolation level. Can someone explain what goes on throughout an Oracle update? And what goes on when multiple read committed transactions attempt to perform a concurrent update t set c = c + 1 where id = 3. What is the result, given c = 1 before either from the transactions, and what's happening using the locks and SCN?

Begin T1
Begin T2
T1:  update t set c = c + 1 where id = 3
T2:  update t set c = c + 1 where id = 3
Commit T1
Commit T2

You are right, this can lock the row no matter the isolation level. With MVCC you will get consistent reads without any locks, however, you still need locks when writing.

The 2nd transaction will watch for the first ones to finish (eg: COMMIT or ROLLBACK) before trying to complete anything. So within this situation the cursor on T2 would "hang" around the update, awaiting T1 to complete.

You'll customize the SCN after T1 commits and the other after T2 commits.