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. during Arabasta arc he appeared always with Akumai, another okama. 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. For being a revolutionary commander, he was arrested and sent to level 5 of Impel Down. Iva and many other level 5 prisoners disappeared into level 5.5, until he aided Luffy in escaping from Impel Down. He also 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 resided in Level 5.5 of Impel Down with Iva.

It is arguable that Sanji was at one point an Okama, though with differences in the manga and anime on whether or not it was against his will. But it is clear that there was a short point in time that Sanji was one.