There's some good info on Ullanor in the new Fear To Tread book.
The main fighting at Ullanor was the Emperor leading the Guard & Titans in a massed steam-roller fight against the orks whilst Horus led the Luna Wolves strike force that killed the actual warlord. I think the other Astartes were fighting on other worlds against the same waaagh, not Ullanor itself? Afterward the death of the Warlord, the waaagh broke up into in-fighting.
Then there was the Triumph, where Horus was made Warmaster and the Emperor retired.
After that various Imperial forces wiped out the remaining orks from the Waaagh - notably the White Scars in the Chondax System (where the Alpha Legion intentionally kept the White Scars bogged down, as briefly shown in the Alpha Legion story in The Primarchs anthology). Of course, orks being orks you can never completely defeat them, but that's their last major part in the Great Crusade/Horus Heresy (although the Ultramarines thought Calth was going to be a muster before attacking orks...)