User blog comment:James12708/Reapers VS. Forerunners VS. Angels from Darksiders VS. Humanity from Halo VS. Council and humanity from Mass Effect VS. Imperium of Man/@comment-4721065-20130112104250/@comment-4721065-20130116091145

You have said yourself that most of the combat will be happening in their City worlds, where the ceilings aren't exactly that high, so some Orks might just be in jumping distance of them 90% of the time.

Even if that isn't the case, Orks have A LOT of Stormboyz, the jetpack-equipped Ork mobs that would make the issue of Onyx flight irrelevant (given that cities' environment provides heaps of raw material for their Meks, the entire Ork force might shift to being Stormboyz: it would certainly be easier to organise than the far more common Warbiker mobs.)

Creating explosions big enough to be visible from orbit is ambigous enough to be almost meaningless, as it is unknown how close is to surface the orbit is and whether that is viewed by naked eye or through lens (not to mention the clarity of the atmosphere & other factors). In short, I'm quite confident that Orks' own mechs (the Gargants, for one, can definitely create explosions big enough, & they don't rely on them as a rule) & anti-tanks will provide a reasonable enough counter before the Orks' ability to repurpose enemy tech & create their own from scrap will kick in.