It isn't OBOII. It is DROIT. The Norman French for a right or entitlement. As in 'Dieu et mon droit' and 'droit de seigneur'. It is simply the motto of the Tunstalls, so sits naturally above one set of their coat of arms.
Peter Mullins used to think he had the potential to be a rather effective parish priest, and has had a spell when his job involved the in service training of other clergy, but after thirty years every new experience makes him realise just what he has missed and is missing.