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.

Okama in One Piece
Eric 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 Eric's frustration.

The first real okama to appear in the story is Bentham, also known as Mr. 2 Bon Kurei, who first appeared as an 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. Even the animals on the island are shown to be okamas.

The ruler of the Kamabakka Kingdom is 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. After the Whitebeard War, Iva returned home along with the other residents of New Kama Land.

Inazuma is one of the okama who resided in Level 5.5 of Impel Down with Iva until he broke out and went to Kamabakka Kingdom.

Newkamas
This is a group of okamas led by Iva, and they were all former Impel Down prisoners. Unlike regular okamas, newkamas claim to go beyond the concept of gender due to that almost everyone of them has changed genders thanks to Emporio Ivankov's Horu Horu no Mi which allowed them to experience life in both male and female bodies. All Newkamas wear revealing effeminate clothing and they are all very strong fighters due to having been former prisoners.

After escaping Impel Down, they followed Iva to Marineford to help Luffy save Ace, even though the war ultimately ended with Ace's death. The Newkama cheered Luffy on for a full recovery, and then went with Iva back to Momoiro Island.

Trivia

 * Newkama is a pun between "new okama' and the word "newcomer".
 * Bentham and Iva are the only known okamas to be able to switch genders on their own.
 * As of yet humans are the only one of the major races to have been shown as okamas.