First of all, to my knowledge, there is no agreement with OCTGN. Nevertheless I don't find anything inherently wrong with some game recommending our site for their support. So the question is really not whether it's okay they recommend us, it's just whether we consider OCTGN-related questions on-topic.
OCTGN belongs to a family of card/board game multiplayer platforms, which also includes VASSAL, Magic Workstation, LackeyCCG, Generic Collectible Card Game and possibly more. I've found some VASSAL-relation questions on boardgames.se, such as this and this, and found OCTGN mentioned there as well, and couldn't really find meaningful mentions anywhere else on the stackexchange network.
So, do we allow questions regarding these platforms? We do allow questions regarding other platforms, such as Steam. It's true Steam is a platform for "pure" video games, not emulated card or board games, but the previous discussion here on meta about whether computer-emulated card or board games are on-topic didn't result in any clear policy.
On the other hand, beyond the links to the questions I've found in boardgames.se above, they also seem to explicitly allow these kinds of questions in their FAQ.
So, are they on-topic or not? I believe that in the driest technical sense they're okay here, but I think the vast majority of users of these platforms are board- or card-game players at heart, so in my personal opinion these questions are a better fit over boardgames.se, simply because of the nature of the community; and for the sake or preventing fracturing, I think we should actively disallow these questions here, and migrate them there instead - provided boardgames.se agree.
And if others agree with me, and if boardgames.se approve, I think it's a good idea to contact OCTGN and recommend that they shift their link to boardgames.se.
TL;DR - I think these questions should be off-topic here and migrated to boardgames.se, provided the community there agrees.