If the orks were to fight against Zerg, it would depend on if it were Zerg lead by cerebrates and also if the overmind was still alive, or Zerg lead by brood mothers, who were created by the Queen of Blades.

The cerebrates have shown to be very good generals (they're giant brains, after all). They intercept enemy radio transmissions, devise new strategies, can communicate with each another, and are all psionically connected to the overmind, who will ensure that the cerebrates will be reformed if slain, so that their valuable knowledge will not be lost.

Under their command, the zerg could easily win against the ork hordes, unless they were lead by some extremely sneaky and experienced blood axe-warboss, who already knows how Zerg fight.

The starcraft 2-brood mothers on the other hand are too inexperienced and don't have really that much combat experience, being more focused on breeding new zerg-strains. Sure, new deadly zerg-thingies can also win the day, but without the knowledge to use them effectively, the queens and brood mothers just aren't that dangerous overall.