A screenshot from Pokémon HeartGold Version and Pokémon SoulSilver Version the summary of a Cyndaquil, including the OT, which is highlighted blue because the OT is a male.
Original Trainer, often shorted to OT, is a term that refers to the Pokémon Trainer who had first obtained a Pokémon.
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An OT can always be seen in the summary of a Pokémon. The OT and the trainer ID number both verify whether the Pokémon has first been caught by the player or if they have received it from elsewhere. With Pokémon Eggs, the Original Trainer is determined when the Pokémon hatches, so if an egg is traded, the OT of the Pokémon is the player who received the egg.
If a Pokémon's OT is different than the player's, the number of experience points it receives in battle is increased by 50 percent, so if the player does not have every Gym Badge, they are at risk of having a Pokémon that lacks obedience; the risk is higher if a Pokémon is transferred from a game with a different language setting, since the experience points increase by 70 percent instead.
At any location where Pokémon can produce eggs, including a Pokémon Day Care, the Pokémon Nursery, or during a picnic, two Pokémon lay eggs more frequently if each one's OT is different, but they also have to be part of the same egg group.
In earlier games, a Pokémon with an OT other than the player cannot have its nickname changed, even if it had never been given a nickname. From Pokémon Sword and Pokémon Shield onward (excluding Pokémon Brilliant Diamond and Pokémon Shining Pearl), the player can give a nickname to a Pokémon with a different OT if that Pokémon has never been given a nickname, but only the OT can change the nickname again.
In the Generation III, Generation IV, and Generation V games, the OT is highlighted a certain color depending on the player's gender: If the OT is a male, the name is blue, but if the OT is a female, the name is red. In Pokémon FireRed Version and Pokémon LeafGreen Version and games from Generation VI onward, the name is not highlighted, but the OT's gender still exists within the data.
The OT of an Event Pokémon is usually based on the subject of the event. The event Darkrai that coincides with the film Pokémon: The Rise of Darkrai has the OT "ALAMOS," a reference to Alamos Town, where the film takes place.