Different boat but I have a Vision 46 with 55HP Volvo with fixed 3 blade prop saildrive. Over 4 months cruising in a variety of conditions including motoring, motor sailing even into heavy seas and normal docking, mooring anchoring and occasional charging etc I averaged 0.7 US Gal/hr. This was calculated logging engine hours between fill ups of the tank. I typically do not run my engine over 2000 rpm.
If the Ocean 38 is a heavier boat that may help correlate a little for you since we may be similar weight....
As stated before I would log engine hours between fill-ups and calculate yourself. If your fuel gauge is anything like mine it is invaluable information to have at hand !