I've never played Star Trek Online, but I hate the idea that the Klingons would become antagonistic towards the Federation yet again. I mean Jebus, make your mind up guys.
I much prefer to think that the Dominion War made them even stronger allies, as really even though they were at peace/allies from 2293, and strengthed by the Enterprise-C incident of 2344, all the way up to DS9 they'd never really fought side by side.
From what we know anyway, there were wars/skirmishes during that time such as the Cardassian border wars, Talarian & Tzenkethi conflicts etc, but I doubt the Klingons ever got involved in those.
I like to think the Klingons were impressed with the UFP in those battles and realised they weren't as "weak & pathetic" as they often made them out to be. And then as Daniels said in Azati Prime, the Klingons eventually join the Federation at some point.