Object
An ability on the stack, a card, a copy of a card, a token, a spell, or a permanent. See rule 109, "Objects."

Offering
A keyword ability that modifies when you can cast a spell and how much mana you need to spend to do it. See rule 702.45, "Offering."

One-Shot Effect
An effect that does something just once and doesn’t have a duration. See rule 610, "One-Shot Effects." See also Continuous Effects.

Ongoing
A supertype that appears only on scheme cards. See rule 204.4, "Supertypes."

Opening Hand
The hand of cards a player starts the game with, once the player has decided not to take any further mulligans. See rule 103.4.

Opponent
Someone a player is playing against. See rules 102.2 and 102.3.

Oracle
The reference that contains the up-to-date wordings (in English) for all tournament-legal cards. A card’s Oracle text can be found using the Gatherer card database at http://gatherer.wizards.com. See rule 108.1.

Outside the Game
An object is "outside the game" if it isn’t in any of the game’s zones. See rule 400.10.

Owner
The player who (for purposes of the game) a card, token, or copy of a spell belongs to. See rules 108.3, 110.2, 110.5a, and 111.2.