Opposition Agent — ruling 1

You must follow the normal timing permissions and restrictions for each exiled card. If one is a land, you can't play it unless you have land plays available.

Opposition Agent — ruling 2

While controlling an opponent, you make all choices and decisions for that player. However, because the control effect is limited to while they're searching their libraries, it's unlikely the player will be allowed to make any decisions other than what to find with the search.

Opposition Agent — ruling 3

You can't have the player find cards in their library that aren't asked for by the search instruction. For example, if the opponent is searching their library for a basic land card, you can't have them find a different card, such as a creature card.

Opposition Agent — ruling 4

If the search instruction states a quality of the card to find, such as a color or card type, you may have the opponent not find any cards. If the search is simply for a number of cards without stating a quality, you must have them find that many cards (or as many as possible, if their library doesn't contain that many cards).

Opposition Agent — ruling 5

While controlling an opponent, you can't make any choices or decisions for that player related to tournament rules. You can't make them concede or have them agree to an intentional draw.

Opposition Agent — ruling 6

The cards found in the search will be exiled rather than be put wherever the spell or ability tells the opponent to put them after finding them. Any other effects the spell or ability has will still apply. If such an effect refers to the found cards, it can't see them in exile.

Opposition Agent — ruling 7

If a spell or ability instructs an opponent you control to search multiple zones including the library, you decide what cards they find in all zones. They'll exile all cards they find in any of the zones.

Opposition Agent — ruling 8

If more than one player controls an Opposition Agent, and another player searches their library, the controller of the Opposition Agent that most recently entered the battlefield controls that player during the search. Because the last ability of each Opposition Agent is trying to exile the card and give permission to play it, the owner of the exiled card chooses which effect wins. However, that owner will be under another player's control, so that player controlling that owner actually makes the decision. In other words, the player who controls the Opposition Agent whose effect applies (and thus controls the opponent) can choose to give themselves all the play permissions unless they're feeling very generous.

Opposition Agent : règles, fonctionnement et interactions (MTG)

Opposition Agent is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flash You control your opponents while they're searching their libraries. While an opponent is searching their library, they exile each card they find. You may play those cards for as long as they remain exiled, and you may spend mana as though it were mana of any color to cast them.

How it works : Opposition Agent

Original English text — Wizards of the Coast / Scryfall

#1November 10, 2020· Wizards of the Coast
You must follow the normal timing permissions and restrictions for each exiled card. If one is a land, you can't play it unless you have land plays available.
No other card is named in this ruling.
#2November 10, 2020· Wizards of the Coast
While controlling an opponent, you make all choices and decisions for that player. However, because the control effect is limited to while they're searching their libraries, it's unlikely the player will be allowed to make any decisions other than what to find with the search.
No other card is named in this ruling.
#3November 10, 2020· Wizards of the Coast
You can't have the player find cards in their library that aren't asked for by the search instruction. For example, if the opponent is searching their library for a basic land card, you can't have them find a different card, such as a creature card.
No other card is named in this ruling.
#4November 10, 2020· Wizards of the Coast
If the search instruction states a quality of the card to find, such as a color or card type, you may have the opponent not find any cards. If the search is simply for a number of cards without stating a quality, you must have them find that many cards (or as many as possible, if their library doesn't contain that many cards).
No other card is named in this ruling.
#5November 10, 2020· Wizards of the Coast
While controlling an opponent, you can't make any choices or decisions for that player related to tournament rules. You can't make them concede or have them agree to an intentional draw.
No other card is named in this ruling.
#6November 10, 2020· Wizards of the Coast
The cards found in the search will be exiled rather than be put wherever the spell or ability tells the opponent to put them after finding them. Any other effects the spell or ability has will still apply. If such an effect refers to the found cards, it can't see them in exile.
No other card is named in this ruling.
#7November 10, 2020· Wizards of the Coast
If a spell or ability instructs an opponent you control to search multiple zones including the library, you decide what cards they find in all zones. They'll exile all cards they find in any of the zones.
No other card is named in this ruling.
#8November 10, 2020· Wizards of the Coast
If more than one player controls an Opposition Agent, and another player searches their library, the controller of the Opposition Agent that most recently entered the battlefield controls that player during the search. Because the last ability of each Opposition Agent is trying to exile the card and give permission to play it, the owner of the exiled card chooses which effect wins. However, that owner will be under another player's control, so that player controlling that owner actually makes the decision. In other words, the player who controls the Opposition Agent whose effect applies (and thus controls the opponent) can choose to give themselves all the play permissions unless they're feeling very generous.
No other card is named in this ruling.
#9November 10, 2020· Wizards of the Coast
In a multiplayer game, if a player leaves the game, all cards that player owns leave as well.
No other card is named in this ruling.
#10November 10, 2020· Wizards of the Coast
You'll still pay all costs for a spell cast this way, including additional costs. You may also pay alternative costs if any are available.
No other card is named in this ruling.