DOPI is the acronym for "Double = 0, Pass = 1." It is a convention used after partner bids 4NT Blackwood but the next player overcalls (usually in a sacrifice attempt). Using DOPI, the Blackwood bidder's partner has the following options available:

X	0 aces
Pass	1 ace
Cheapest bid over the overcall	2 aces
Second-cheapest bid	3 aces
Third-cheapest bid	4 aces

If opener rebids 5NT, he is making a Blackwood query for kings. All other rebids by opener are natural signoffs.