An Ork mob has to check morale and test for pinning from barrages and snipers in the same way as any other unit. However, if the mob fails a test it will immediately 'check size' by rolling 2D6. If the score is equal to or less than the number of Orks (not including Gretchin) left in the mob then the Orks carry on, confident in the belief there are enough of them left to win. Their courage is bolstered by their comrades, spurring them on to battle. This means that an Ork mob of twelve models or more will always fight on, regardless of any casualties it might have received.
Mobbing Up
Orks falling back can attempt to regroup as normal if there are enough of them left, but this rarely happens as Orks will only withdraw once most of them are dead. However, an Ork unit of any size can attempt to join another mob or in other words 'mob up'.
When Orks fall back they can move towards any Ork mob that lies behind their own position. If any withdrawing Orks get within 6" of the new mob once moves are complete then the withdrawing mob can attempt to join up with the new one. Make a 2D6 roll against the Leadership value of the mob which is falling back. If successful the two mobs combine together (mob up) and the previously withdrawing mob can immediately move up to 6" so they are in proper formation.
Only Ork foot troops will mob up in this way – Slavers and Meks in charge of Grot mobs or Big Gunz, Stormboyz and warbikes may not mob up.
Victory Points after Mobbing Up
A mob of Orks which mobs up counts as destroyed for victory points purposes. An Ork mob which gains additional Orks from another mob joining up with them is not worth any more victory points than it was before, it will still use its starting strength and points value for working out victory points.
Errata — Chapter Approved: Codex: Orks Q&A (v1.0): Even a mob that automatically fails its Morale check (eg against a fearsome opponent) still gets to make its mob size check afterward — the size check always follows a failed test, automatic or not. Mobbing up is only checked "once moves are complete" — any pursuits, crossfires, or other effects that resolve during the move happen first and can still wipe out a fleeing mob before it gets the chance to mob up. If a falling-back mob's full move would carry it off the battlefield edge, it cannot attempt to mob up even if it passed within 6" of a friendly mob partway through that move — a move that ends by leaving the table is a complete move off the board, and mobbing up requires the move to be complete first.