Question

Does OleDbTransaction auto rollback if I don’t call commit before it gets disposed?

Answer 1

See here for the MSDN documentation on the details.

The transaction must be committed, otherwise the staged changes do not get committed.
Upon error you should call the rollback of the staged changes. This quote from the documentation answers the question. Yes it does get rolled back.

The transaction can only be rolled back from a pending state (after
BeginTransaction has been called, but before Commit is called). The
transaction will be rolled back in the event it is disposed before
Commit or Rollback is called.

http://msdn.microsoft.com/en-us/library/93ehy0z8.aspx