So this is mostly just to sate my own curiosity. I made my own mass combat house rules for Black Crusade and they work equally well with any FFG 40k game (and in fact only minor adjustments are made to get it to work with, like, d20 or something, though the factions are all 40k-specific). After I finished a major update, I decided to post them to Only War. Now, when I posted them to Black Crusade I mentioned how they were actually different from the mass combat rules for that game: They scale up to higher than the Tome of Blood rules, and work much better for fighting orks or 'nids or other very aggressive enemies than the Imperium-focused Tome of Decay rules. Since I was exhausted when I posted the updated rules to the Only War forum, I never bothered explaining why anyone would want to use my rules instead of the Only War mass combat rules. In fact, I can't even remember what the official mass combat rules are, because I've never actually played that game.

So I decided to look them up. And it turns out they...Don't exist? Are there really no rules for combat on a larger scale than the formation rules (which only really work up to numbers in the low hundreds on either side) given in Enemies of the Imperium? I don't have every book, but Google isn't giving me any results but house rules.

tl;dr Does Only War even have mass combat and/or military campaign rules? And if not, why on Holy Terra not? Even just copying the Tome of Blood and Tome of Decay's systems over would've been better than nothing.

One of the Rogue Trader supplements had mass combat rules, too.

For Only War, my guess would be that they're just waiting to publish something like this in another supplement.

However, it is also possible that the studio believes the game not to warrant such rules due to scope. Both Black Crusade as well as Rogue Trader have players actually in command of such huge formations, whereas Only War only really supports company-level command (with the Commander speciality in HotE), where tactical gameplay is tended to with Formation rules. Player characters are not expected to take command of multiple larger units as this would shift the scope of adventure away from squad combat (arguable the game's focus) and into some fortified bunker, so perhaps the designers intentionally omitted actual mass combat mechanics.

Personally, I would still include them, but rather as a mechanic to manage background narration for the GM instead of giving players a turn-based strategy game of sorts - meaning, a system that keeps track of where the various units in a military campaign are and what they are doing, so that the players' missions can develop "naturally" out of this strategic element (including the players' actions influencing the greater picture) rather than the GM having to "wing it".

A bit like how GW's Planetary Empires system worked as a tie-in to generate skirmishes for the Tabletop.