You will at some point need to do something serious about its tuning if you want this content to be seen by the population and not be a complete waste of resources.
It not only suffers from being a 40 man, which was a bad idea to begin with (WoW already had a much larger playerbase, and it still had to retire 40 mans for the simple reason of logistics and guild management). But tuned as it is, middle tier guilds will get to clearing Ohmna, and hit this massive brick wall of difficulty and inconvenience that will likely end in a bunch of the raiding population canceling their subs.
It's a really impossible situation you've created, though. Either you change it to 20 man and keep the difficulty, or you have to nerf this instance considerably for the additional trouble of a 40 man roster being worth the effort. The latter means some players will be pissed because they had to clear it at its current difficulty, but other than introducing a easier version of it you will have ended with a raid that a very tiny percentage of the population will have enjoyed. And I'm worried you've yet to learn from other MMO developers' experiences that this is not a good thing.
It's fairly clear that with the current population of the game 40 mans are not feasible, and what's worse is that with the current playerbase left, very few people will even have enough of a pool of able players to start even attempting this content.
I don't understand developers who choose to throw away money that they could otherwise see added to an increased development budget by making a game that is so repellent and inaccessible to a large part of its playerbase. These kind of large person raids don't really work in the current climate of the genre anymore unless you have something akin to LFR, and I doubt people want the opposite extreme which is trivialized mechanics.
Keep to the 20man raids and overhaul this raid instance to 20-25 person mechanics.