Managing your OID rather than letting NHibernate manage it actually makes like significantly simpler. I use a base persistent entity class that assigns a guid for use as the OID and contains an integer version field so that NH can do the necessary transient / persistent checks (as well as allowing for concurrency management).

IMHO letting NH manage your IDs, while correct from a “pure” POV (OID is a persistence concern, not a domain concern), causes more problems than it solves.