While there are no specific canon 'Food Chains' out there...there is some information that will help you figure this out.
In the back of the DMG is a series of Monster Lists. The ones I'm speaking of specifically are the ones that list Monsters by Biome (page 302).
In order to find a predator that would potentially hunt Owlbears, all you have to do is find a Biome containing Owlbears, and look for something carnivorous that sits higher on the CR tables than it does...or something that operates in packs and is sufficiently hostile.
To give an example...the 'Forest' biome includes the Owlbear, which has a CR of 3. Also on that table you have the Gorgon (CR 5) and the Grick Alpha (CR 7). Both of these are listed as predators in the monster manual...and thus could be feasible predators for an Owlbear. Then, of course, you have the more intelligent monsters that could hunt an Owlbear in groups, but if you want a 'Natural' predator...I'd look for unintelligent creatures.
That said...the MM does seem to specify that Owlbears are an apex predator. There may be monsters out there that are tougher than an Owlbear, but most of them leave Owlbears alone because, well...
The owlbear's reputation for ferocity, aggression, stubbornness, and sheer ill temper makes it one of the most feared predators of the wild. There is little, if anything, that a hungry owlbear fears. Even monsters that outmatch an owlbear in size and strength avoid tangling with it, for this creature cares nothing about a foe's superior strength as it attacks without provocation (MM 249)
So, for this specific case....finding something willing to tangle with an Owlbear that is a natural predator might be pretty hard.
Hopefully this is helpful