Dining at the O is a term for the act of performing analingus on a female or femme-identifying person.
It is most commonly used within the sex industry. For example, you may see "dining at the O" listed as an item on the menus of sex workers and in the field reports of clients of the sex industry. In the sex industry, when a client requests dining at the O, they want to perform analingus on a female or femme-identifying sex worker.
However, as some aspects of sex work have gone more mainstream, this term has also entered the common vernacular. People may use the term dining at the O when telling someone they performed analingus on a female or femme-identifying partner.
The term dining at the O is a spin-off of the term dining at the Y. The term dining at the Y, a colloquial term for cunnilingus, originates from the shape of the female genital area. Similarly, dining at the O gets its name because the anus can look like the letter O.
Dining at the O is not used to refer to the event of a sex worker performing anal sex on a client, no matter what the client’s gender identity.
Dining at the O is sometimes shortened to the acronym DATO.