The Orks would need to scientifically discover a way to sterilize the very planets they fought on (something they aren't particularly good at), or else the microscopic spores left behind by the Tyrannids would become parasites on the Orks, anything from burrowing tapeworms to Tryannid bacterial agents and toxins. Not only would the world itself revolt against them, it would infest the Ork race for centuries on the microscopic level and when these creatures enter the cerebral tissue of the Ork (who would be powerless to stop their slow advance through their bodies without advanced medicine, which they don't have) it would become a battle that the Hive Mind would end up winning because its effects are far stronger than the psychic link of the WAAGH. If that happened not only would the Tryannids gain control of many Orks simply because of brain bugs implanted in them separating them from the rest of Ork kind because the Hive Mind can smother it like warp energies with the Shadow of the Warp, they gain the abilities of Orks themselves - rapid spore based reproduction, the ability use technology, and apparently according to some people championing the Orks don't need any resources to survive, the ability to live without actually being alive. Can somebody please explain to me how a living creature can grow without in taking any type of fuel? I know Matt Ward didn't design the lore behind the original Orks, so there has to be a logical explanation to this conundrum. When you figure it out, consider that the Tyrannids are hands down the most efficient at doing the most basic, and yet hardest to sustain thing in the galaxy: Living.

Back on track, many people have been saying the Orks would adapt to fight the Tyrannids. True, but that's not the same as evolving, which occurs over centuries and involves hundreds if not thousands of adaptations. Not only this, the Tryannids can evolve much faster than any other known race in the galaxy, and if there's one that can do it even quicker, you can bet that'll change once the Tyrannids eat them. Which leads me to my next point, Orks can only Loot planets. Tyrannids consume them entirely. Any planet lost to the Green Tide only becomes a greater asset to the Great Devourer, but vs versa, any planet lost to the Tyrannids, is gone forever. Not only this, but the planets that are devoured give far more resources to the Tryannids and much faster than the Orks. Metals and stone all provide valuable minerals that are invaluable to the Tyrannids and are leeched, if not scoured clean along with the biomass of a fallen planet. It takes time to build, mine, refine, and then finally forge weapons and materials Orks would need to fight Tyrannids, because unlike every other force in the galaxy, the Tyrannids deny them of their greatest asset, the ability to Loot and continue the war effort, because they aren't particularly good at making their own weapons. I know someone is going to post the pic of a Mekboy riding a looted Carnefex, and apart form its virtual impossibility in the lore, the materials of the Ork's riding platform are in relatively short supply considering the other races (except for Chaos in the Eye of Terror) would either all be dead or consumed for this to happen. In short, the vast supplies of the galaxy favor the Tyrannids way more than they do the Orks. Therefore Tyrannids are far more adaptable than Orks are, and given the time constraints of pitting two enemies with virtually limitless power, it's going to be a very, very, very long war to sort out. Well beyond the time that it's going to take for the entire Tryrannid race evolving to become an aethema to the entire Ork race no matter what battlefield adaptation it can think of as it eventually runs out of things to Loot as the Tyrannids continue to destroy them and everything else.

Going back to the whole living thing I mentioned earlier. The Tyrannid race itself is geared towards the extinction of all other life. It is the dominant life form of the known universe even beyond the ambitious and formidable race that are the Orks. It achieves this by evolution, brutal efficiency, the permanent denial of others to gain resources, and even more importantly: unity of purpose. Orks strive for war. That's pretty much it. Even under one banner there will always be infighting, rivalries, brawls, and even civil wars to prove to each other their superiority. It's basically how one get's promoted to sergeant. You kill your sergeant. It's not efficient. It's not methodical. It may be effective in the 40k universe as it weeds the Ork race to be tough and strong, but against the Tyrannids it's suicide. Every available asset must be deployed at all times against the Tyrannid race or else they will steamroll over you, consume you, and then use your chummy bits to make more chummy bits, which in turn makes... yeah.

Though the Tryannids can be splintered momentarily from the Hive Mind, they will never fight against themselves as soon as a synapse creature regains it's link to the Hive Mind and is no longer feral. The Orks on the other hand would never be united long enough to fight a war that would last for millennia or more. It's just not going to happen under any circumstance, because Orks are Orks. They like killing. And if they're aren't killing Tyrannids, they're killing each other.