The Disagreeable Order of Grouches (D.O.G.) is an organization of grouches modelled after real-world guilds and societies typically offering exclusive membership to individuals who meet specific criteria.
Its members — who include Oscar the Grouch, having mentioned them several times over the years — are issued a book of rules and a ring, and they wear a fez emblazoned with the order's acronymn. Other fellows include Hubert Rotten (Episode 0072), Charlie Rotten (Episode 0174), and Stanley Stinker (portrayed by David in an imaginary scenario from Episode 0596).
Oscar's first affiliation with the club occured in Episode 0042 when he travelled through several states to attend their convention, an event which serves as host to many grouches from all around the world. A smaller meeting was held locally in Episode 0301 when Oscar ordered provisions for some of his affiliates. By Episode 0050, Oscar has begun recruiting for D.O.G. when he gets into an argument with a delivery man and invites him to join. He later offers membership to a non-grouch in Episode 0554 after Gordon gets grumpy with everyone for mistreating his new car.
Their theme song was written by Oscar in Episode 1174 with some help from the kids, and titled "On Top of Old Smelly" (to the tune of "On Top of Old Smokey"). Oscar's efforts to support the club have been recognized in Episode 0146 when he was awarded a ribbon for being the "handsomest grouch of the year" (as an insult) and in Episode 0476 when he earned the Grouch of the Year Award. The honor was bestowed upon him again years later in Episode 2081 when Grundgetta (now serving as D.O.G. president) grants Oscar the prize after having been assigned as a judge for the contest himself.
However, Oscar's place with D.O.G. wasn't always in good standing. When he let Buffy and Cody live in his trash can for a stretch of episodes in season 12, the order called his membership into question. In Episode 1539, a fez-topped representative conducted an inspection of the situation, temporarily stripping Oscar of his position. Buffy proved herself to be just as much of a grouch as Oscar, resulting in his readmission as well as a ceremony inducting her into the club.