I've made a TT druid, core rules only, and as she leveled up, she had to go the route of weird and wacky animals. Lemme see...
Cave bear, Sabretooth, Giant scorpion and Killer whale - and now Water elemental.
Owlbear is better than Sabretooth, although doesn't have the stealth nor the at-will 20' jump. But no better than the scorpion, certainly doesn't have the control. Killer whale is obviously most powerful but waterbound.
Getting it at level 6 is about right. By memory it's possibly a level early, but that's quibbling. It's little more than a bear upgrade and regardless, gets overshadowed later by any elemental form.
Lorewise, the 1E MM speculated whether the owlbear was chimera or beast. I guess they've decided despite its monstrous origins, it's become a stable enough creature within nature.