How do you get rid of this beast ?
At difficulty lvl "normal", this thing kills any opponent in two hits, and wipes my entire team on its own in a bunch of rounds.

What is weird is that on my 1rst run, this owlbear did not attack us and stayed in its pen but now, I don't find a way to keep it far from the battle - not even mentionning a way to defeat it once it's gone wild.

You need to either tame the owlbear or sabotage the lock.

best option in terms of exp is first calm it with nature and then kill it with stealth. all done through dialogue.
killing it is a challenge but is doable too. web it and pepper it with arrows and spells is decent enough solution for that route.

Nature check is quite difficult. The safest option is to lock it up. You can leave it outside but there are equal chances it will go after bandits or after you, in the later case you're toasted (at least at level 4 with the standard companions - no web). Especially in a no reload game. In a standard game you could just reload until you pass the desired check.

Spoiler.

In my case, my owlbear rescuer ran directly up the ramp to melee Staglord. Owlbear pursued but had trouble keeping up, around the corners and past the random fighters also chasing me. Owlbear remained my dedicated foe and a dedicated member of Staglord's team. Wasn't that hard to kill after Staglord's guys were thinned out.

-edit- I left out the reason I freed the owlbear: I thought it was a trapped wild animal that would attack whoever is closest. I was wrong, but my planned assault went ok anyway.

I had Linzy sneaking around scouting. When she checked the owlbear's pen, there was an option to jam the lock or something like that which meant the gate couldn't be opened. She did that. The first group with my wizard main character, just left after dealing with rest of place. The second group with my druid main character checked the owlbear before leaving the place and there were a couple of options. I chose the kill it by stealth option and succeeded.

I have several other characters that I am trying out in the first chapter and I will probably try some of the other options with them.

Does anyone know how to get the owlbear to fight with you against the Stag Lord? I read some walkthroughs that suggested it was possible, but if you set the owlbear free after taming it, it just wanders off. If you choose "This owlbear will help us fight the Stag Lord," it remains in its pen and you can no longer interact with it.

As far as I remember, that's how it is supposed to be. It becomes non-interactable, and joins the battle when it's time.