"firstly check head for warpage"; then my suggestion would be take the head to a engine reconditioner, get it fully serviced & pressure tested.
it appears to be a bit of a oil burner by pics, maybe give the bores a hone and put a set of rings & bearing in it as well( y apart).would last forever then.
oil in water occurs when; cracked head/blown head gasket(head warpage)/crack in bore(highly unlikely)or if auto trans (radiator my be cracked). cheers ernie, welcome to the site.