I have a minor bias concerning this rec, because I've become one of the two betas for it. However, I used my elbows to shove aside other volunteers once I read the first two chapters, so I'd say beta bias is secondary.
Heart in a Headlock is at first glance another story in the Alpha/Beta/Omega universe, normally a 'verse I avoid unless a slash partner-in-crime forces me to read it. In this case, I doubly would have avoided it because it's also a work in progress. However, I obeyed a rec and am now in love.
I personally don't care for ABO fics because they tend too much to follow a hyper-stereotypical male/female relationship where one or the other of my favorite slash pairings is basically a girl with a dick -- and a 50s stereotype of a girl at that, only with more explicit sex. These fics generally make me EXTREMELY crabby and unable to relate -- if I wanted to read het slash, I'd read it, and if I wanted to read ridiculous het slash, I'd read 60s Harlequin romances.
(For those who haven't encountered the verse before, Alphas stand in for males and are dominant, Omegas stand in for women and are submissive, Omegas go into heat at which time they can't resist mating with an Alpha, Alphas smelling an omega in heat can't resist mating which looks awfully like rape. and then there are Betas, not subject to the Alpha/Omega dynamic, who stand in for normal people. Oh, and somehow along the way, Alphas have acquired hormones to dominate Omegas and the penile characteristics of dogs and wolves. If you want more info, google.)
Heart in a Headlock is different. Sweetandsharp, the writer, starts with the premise of the ABO universe and asks , "How would it feel to live in such a world?" So we get Merlin, a college professor who keeps his Omega status secret and is doing research on chemical blockers -- various medications designed to give the sexual denizens of this universe characteristics of other genders, or to disguise their own. He is a closet user himself, so is believed to be a beta.
Unfortunately, he has a very rare condition which means that he can only mate with a particular alpha who has the same condition. This, inevitably, is Arthur, an arrogant alpha who doesn't recognize his own condition and has been looking for a mate since puberty, which has improved neither his disposition nor his social skills. The two meet at a "run" designed for omegas and alphas to find each other, though Merlin is going ostensibly purely for research reasons and remains beta-disguised. Arthur does not recognize him as an omega, let alone Arthur's mate, and trouble ensues.
A romantic comedy which includes some serious research and lecturing concerning the endocrine system and cultural analysis of male/female stereotypes may not be to everyone's taste, but it certainly is to mine. In fact, I can well imagine sweetandsharp's story someday being turned into a Tony-winning musical, were it not that those who write fanfic are doomed to our own closets. There's a solid plot concerning the meds and their effects, but that's almost as much in spoiler territory as the writer's mischievous list of participants in Merlin's study of blockers, all of whom come from a wide range of fan canon.
At any rate, do check it out -- even those of you generally committed to H/D forevah. It'ss well worth reading, and a thoughtful introduction to the ABO verse.