The following special rules apply to all Ork units except where noted.
Due to the wide variety of wargear and the Ork Mobbing Up rule it is entirely possible for some units to include models with different armour saves. Because of this the normal casualty removal and armour save rules are altered slightly.
The opposing player rolls to hit and wound for whichever unit he is firing with as normal. However, when the Ork player makes armour saves before removing casualties from a unit that has mixed armour saves he uses the best armour saves as long as the Orks wearing that type of armour are in the majority (ie, they outnumber Orks with worse saves). This represents the heavily armoured Orks making more obvious targets and the lighter armoured Orks using them as cover! Any casualties removed after saving throws have been taken must come from amongst the most heavily armoured Orks first (ie, the ones with the best saving throws take the hits).
If heavier armoured Orks are in the minority use the worse armour saves and take the casualties from the lightly armoured Orks instead.
For example: A Warboss and his bodyguard of five Nobz suffer 6 wounds from enemy fire. The Warboss and three of his bodyguard are wearing mega armour so the Ork player rolls his saving throws using the 2+ mega armour save (as the models with mega armour outnumber the other models in the mob). Unluckily he fails two saves and 2 wounds are suffered. Because the Nobz have 2 wounds each, one mega armoured Nob is removed as a casualty. If the enemy fire had an Armour Penetration of 2 or better, three mega armoured Nobz would be removed as casualties (ouch!).
Errata — Chapter Approved: Codex: Orks Q&A (v1.0): This rule only concerns Armour saves. A Cybork Body's Invulnerable save is ignored entirely for Mixed Armour purposes — saves are taken using whichever Armour save type is in the majority in the mob, regardless of how many models also happen to have Cybork bodies. Ammo runts can't be chosen as casualties and so never make saves themselves; they're simply ignored when working out which Armour save type has the majority. If there genuinely is no majority (an even split, or several different save types each represented equally), use the best Armour save present in the mob.