Okama

Okama is Japanese slang for homosexual or transvestite. Literally meaning "a pot, a kettle" this word, always with the honorific prefix "O-" refers to a gay man, especially one who is viewed as effeminate or a drag queen. Can be pejorative. The word originated in Edo period slang for the anus. In current use.

Okama in One Piece
Erik the Whirlwind is a filler mercenary who wields the Kama Kama no Mi. Due to the "Kama" part of his powers, Luffy keeps calling him an okama, much to Erik's frustration.

The first real okama to appear in the story is Bentham, also known as Mr. 2 Bon Kurei, who first appeared as the enemy to the Straw Hat Pirates, but ended up being friends with them. Bentham is a practitioner of the Okama Kenpo martial arts. He also wields the Mane Mane no Mi, which allows him to physically transform into other people, even other genders.

Kamabakka Kingdom is an okama-infested kingdom, which Sanji has the greatest misfortune to be sent to.

The former ruler of the Kamabakka Kingdom was Emporio Ivankov, Iva for short, currently resides in Level 5.5 of Impel Down. The reason for his arrest is unknown, but he has the power to manipulate hormones of his targets through his Horu Horu no Mi, which can include changing genders.

Inazuma is one of the okama who resides in Level 5.5 of Impel Down with Iva.

Trivia
The two most prominent okama, Bentham and Iva, have the ability to change himself or others into opposite genders, respectively.