Originally posted by Isaac King:
The vast majority of competitive players in the US use “ok” to mean “resolves”. You should always investigate whether it's been used differently prior in the match (especially if you're in a different culture that may have different customs), but our rulings should reflect how Magic is actually played. Defaulting to “ok is just ok, it doesn't mean anything” allows players to angle shoot by making the opponent think a spell is resolving and then not being held to that statement by a judge.
Especially in this scenario, NAP didn't just say “Ok”, AP asked “Ok?” and NAP confirmed it. I think that makes it pretty clear that the spell is resolving, since AP does not need NAP's permission merely to cast it.
However, AP did not declare 0 targets, AP missed that step altogether. I'd want to investigate to see how well AP knows the rules and determine if I think they were trying to trick NAP into losing their chance to respond. If I think it was an honest mistake, I'd be giving AP a GRV and backing up to casting the spell properly.
At MF Kansas City I played 4c Dreadhorde in the main event, and my standard procedure for casting this spell would be to tap six mana and put a Command the Dreadhorde on the table, then pause and look at my opponent. I'm fully aware of how the card works, but I did this for a few reasons:
1. I don't want to pick up my opponent's cards (out of their graveyard) without their permission, and its often difficult to know what valid targets are in the opponent's graveyard without picking them up.
2. I want my opponent to confirm that I tapped the correct colors of mana for the spell (which in a four color deck is not trivial)
3. I want to confirm my life total with my opponent before choosing targets.
4. I want to see if my opponent will simply concede without me having to spend several minutes choosing targets and resolving the spell and associated explore triggers.
My point is that just because you're casting the spell without picking targets doesn't mean that you're trying to sneak something past your opponent; there are several other reasons why you could be doing this. In particular, with this spell, its very unlikely that you would try to trick your opponent in this way because the “correct” targets are almost always known to both players- since its mostly public information- and its much more likely that the player is sacrificing technically correct play to make the gameplay easier..
I understand that my situation is different from the situation in the OP (I didn't explicitly ask “is this okay”) but I would still be wary of a line of questioning that resolves around whether a player knows how the spell “technically” works, versus asking what their intent was.