I'm sorry if this has been answered before but I failed to find it in the rules, No Retreat wiki FAQ and in this forum...
yes I think it was asked before, but there is a lot of topics posted here!
Do I understand correctly, that the OOS marker is treated in multi-hex combat in all aspects like all other factors in the TEC? For example, if the attacker attackes both a supplied hex and an unsupplied one, the defender can choose which one is checked for the TEC effects including terrain, tank bonus AND supply?
Yes this is correct (this is why the OOS marker is part of the Terrain Effects Chart). But you do chose ONE hex only, you cannot go from hex-to-hex fishing for bonuses. take one hex and apply all the shifts from that one hex.
Also what happens when two units stacked in one hex - one OOS, the other in supply - get attacked. Does the attacker gain the two column shift or do all of the units need to be OOS? I haven't encountered this situation yet, but as it is possible and I don't know the answer it probably WILL happen next time I play
Yes the attacker will gain the bonus.