If you can’t see your target, make an attack roll at disadvantage
The rules for unseen targets say:
When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see. If the target isn’t in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target’s location correctly.
Nothing here about a dex save. The orc attacking you rolls their attack at disadvantage, still trying to hit your armor class.