I am trying to set up a campaign for my group that deals with a Tryanid invasion and I was wondering if anyone has tried to use the stats from the other RPG lines or if there are any balanced conversions for Only War.

I would use the stats from Deathwatch and use only the lower tier Tyranids, those are dangerous enough.
Upper tier Tyranid will rip you pc's apart.

Depends on what the players are armed with themselves;

Termagant/Hormagaunt packs make a nice opponent for the guard - the guard can outfight superior numbers (a bit) but the fact that they can't be pinned and are very fast makes up for this.

Tyranid warriors are very scary for a guard squad - they're up there with space marines and can tank massive amounts of fire.

A carnifex or tervigon is beatable for armoured units - especially a vanquisher. If they're not in a tank, then a heavy weapons squad might do it. Otherwise you're really just feeding them lunch.

Yeah, Nids are built without some of the bells and whistles, those bein in this case the little things that make them less engines of doom, and more people. Everything about them is "kill this, eat that", and they don't usually get scared, succumb to arrogance, or allow themselves to be negotiated with; you have to kill them, and they will try to kill you.

One thing, and I don't know how most people's average games of Only War are run, will depend on if the game is an active battle, with a large army, and your players are a veteran unit among them, or if they are usually more independent, going off to sneak, recon, and eliminate a key target, sort of like an Inquisitor's Acolytes. If they can benefit from the nearby hordes of their fellows, some artillery company fire, and such, a lot more of the Nids can also be used, but if they are more "one unit, the focus", and often go off to do things, away from their command staff, then big Nids are best avoided (and maybe try to feed the Commissar to them, if he starts itching his bolter when you don't just charge the Zoanthrope like it's a good idea. Some little Nids are certainly doable, and like Zerg in Starcraft, I imagine many Tyranid invasions starting with little foothold organisms, but full on hordes of Termagants (the CC/assaulty ones) can hurt, and Genestealers are really just blenders with feet. Bigger bugs (Zoans, Fexes, anything with "Tyrant" in its name) are better felled by tanks, or massed heavy weapon teams (h. bolters, plasma guns/cannons, anything with good range and Pen 6+). They are a nice example of things that the Guard have to fight as the Imperial Guard, that is an army, rather than one Seal Team who might be the best of the best, sir, but they weren't trained for this crap.

I don't want to say your/a team of OW players CAN'T fight Nids; but it is a strategy-heavy attempt, and they will need some good stuff. It won't be easy, and I kind of feel that's why they hit DW, rather than OW; Space Marines are supermen for a reason.

I imagine a tyranid game having a zombie apocalypse feel... not in the "shambling hordes of walking dead" sense but in the "the world has been/is being destroyed and hordes of remorseless, flesh eating monsters are swarming out of the shadows to eat you" sense. A lot of the roleplaying would involve the kind of desperate, survival-oriented insanity that comes with the whole apocalypse setting.

And unlike many zombie movies, the Tyranids will be a lot harder to get rid of; they contaminate the world, maybe in a way similar to Orks, and there is no cure for Nids, nor often some truly centralized hub of their activity. There's always more out in space to come and take their place, too.

I would give out lots of Insanity Points. Probably give sufficiently large numbers of minor Tyranids a Fear rating.

Right now I'm honestly paying serious attention to this thread, mainly because the OW campaign I have right now is the regiment along with about four more regiments have been sent down to suppress a cultist uprising. However in the first session they've figured out it's a Genestealer cult. So while I've not thrown any genestealers at 'em, I have thought about the possibility of maybe running weaker versions than what's in the DH or DW books along with maybe an occasional Lictor depending on what the 'nid generator' I've got working out on.