Over My Dead Bodies allows you to attack or block with creature cards in your graveyard. If you do, for the duration of combat, the attacking and blocking creature cards are treated as if they are creature cards on the battlefield which means they can be affected by spells or abilities that would affect creatures on the battlefield. That means, for example, if you have a Lord of Accursed on the battlefield (“Other Zombies you control get +1/+1”), all your attacking and blocking graveyard creatures (which Over My Dead Bodies makes into Zombies) would get +1/+1 while in combat.