I imagine the Hereticus as a distant third
Unless Obliterator Cults have been completely retconned from the background material, (and given GW's current track record, I wouldn't be surprised if they had,) then I'd think that the Ordo Hereticus would take a great amount of interest in the Obliterator Virus, assuming of course that it is a virus.
Generally speaking, though, I always imagined that an Obliterator was simply the name given to any individual who could manifest weapons and armour from warp-energy, regardless of how they actually managed to attain such powers. Some might be the victims of a chaos sorceror's curse that turns them into beweaponed thralls. Some might be created through a person wearing daemonically possessed armour, and using daemonically possessed weapons, all of which suddenly decide that they want to fuse together into one unholy abomination. Some might be the victims of some kind of obliterator virus, (and personally, I've always thought that 'obliterator virus' was an Imperial terminology for something that isn't well understood. I always imagined that the Obliterator Virus was more like a swarm of artificial self-replicating nanites rather than an actual virus).
But anyway, to bring myself back on topic, I think that the Hereticus would take a great deal of interest in Obliterators due to their tendancy to start cults within the imperium. Since the Hereticus' mission statement is to wipe out the enemy within, this would mean that Obliterator cults fall into their "pseudojurisdiction."
Also, since the Hereticus is the largest ordo by far, I wouldn't be surprised if the vast majority of Inquisitors with experience in fighting the Obliterators belonged to the OH.