Being able to kill Azuregos is no small feat, especially considering the strength of your guild and how young you guys are, relatively speaking. Your progress in the Molten Core is also highly commendable, what with your lack of priests and gear. So why hasn't there been a concerted effort to buckle down and kill Onyxia and Ossirian? Onyxia should be on farm status for a guild like yours, and Ossirian, with enough practice and dedication, should be like-wise. As a casual observer of Simple Math's progress, I cannot figure this lack of progress out. There is absolutely no reason for this sort of stagnation, and know I am not saying this in a mean-spirited manner, but rather out of concern and confusion. Are you going to let some overgrown whelp and a Sesame Street reject birdman stand in the way of moving forward?
With a strong core 20-man raid and 15-20 warm bodies that know enough not to frolic in the whelp rookeries, you should be able to kill Onyxia. Your tanks are talented enough to dance the fears, your healing pool has enough skill to avoid pulling aggro and still keep everyone afloat, and your dps is competent enough to handle whatever else may stand in your way. Get everyone keyed and get in there, for goodness sake! There are fancy hats and other such things just waiting to be looted, if only you guys pushed hard enough. Onyxia is not serious business, not for your guild.
Some might say that downing Ossirian is beyond your current abilities, and may question the wisdom of "wasting" your time trying to kill him. But that could not be further from the truth. All the Ossirian fight requires is practice and a solid routine that can be pefected within only a few attempts. Don't let the issues of gear and difficulty stand in your way. You took the fight to the Molten Core with lacking numbers and equipment, and look at your progress. Your members burn through the battlegrounds, going head to head with people who have gear that is in every way superior to yours, and still manage to give them a run for their money. Ossirian should not be a problem, if only you were to buckle down and get somewhat creative with your approach. Have faith in yourselves. The same thing goes for the Sulfuron Harbinger fight. Get creative, and get it done.
I hope that Andorhal treats your raid better than Frostmourne did, and that you continue to enjoy the game and progress along at a nice pace. You guys deserve it.