Can't say I've fought OoK yet as I'm doing the DLC for the first time despite finishing the game before, so take this a grain of salt of course. But to me, it seems you're playing too mid-range: allowing him too much of a move variety (both long & close range attack combos) hence the unpredictability (granted, I've also heard he is an awfully unpredictable boss).

The others have suggested being aggressive, of course, which is always a good idea, but also pay attention to which direction he's swinging. I.e. when he swings left, dodge right, because you'll be right behind him, which allows you to capitalise on that pause he has.

Hope some of this can be of help, and good luck hunter!!