• Flex Alexander as Flexter Alexander "Flex" Barnes a.k.a. Flex Washington (seasons 1–4; recurring season 5) – Flex was born to Richard and Eunice Barnes and is the oldest of three children. In high school, Flex met Nicole, who became his first serious girlfriend. Before that, Flex had a reputation with women that caused many fathers in his neighborhood to dislike him. At age 18, Flex got Nicole pregnant as a senior in high school and they got married. She gave birth to a baby daughter named Breanna when they were 19 years old. A year later, Flex and Nicole went their separate ways when Flex pursued a career in the NBA playing for the Los Angeles Clippers, New York Knicks and Orlando Magic. During a game, however, he blew out his knee and was sidelined. Luckily, he had a communications degree to fall back on (which he admitted he tried to get twice) and became a sportscaster in Baltimore for television station WYNX News 3. After his daughter stays over for two weeks, Nicole springs the news that she got a job in Nova Scotia and wants to take Breanna with her. Flex unwittingly volunteers to take care of Breanna, though it may hamper his bachelor lifestyle. After his divorce from Nicole, Flex had taken on a player persona and even had a 30-day rule, vowing not to date a woman for more than 30 days and run the risk of their connection becoming a romantic relationship. He broke that rule when he dated Natalie, Breanna's art teacher. However, they broke up a year into their romance because she wanted to be a good stepmom to Breanna rather than have children of her own. A year later, Flex began dating Danielle, a psychotherapist who initially did not want to date him because Flex and Brianna were her clients. Flex and Danielle eventually began dating and later became engaged. Around the same time, his show The Flex Files became syndicated. In season 5, the show's premise was revamped and Flex was phased out of the series, but he appeared occasionally as a recurring character. He would fly out to California to visit Breanna and to also make sure Arnaz was not trying to have sex with her.
• Kyla Pratt as Breanna Latrice Barnes – Breanna is the daughter of Flex Washington and Nicole Barnes, born when the two were 19. When Breanna finds out that her mother Nicole got a job in Nova Scotia, Breanna comes up with the idea to have Flex take care of her. During her stay with Flex, her best friend Spirit and she joined the cheerleading squad, which she initially would have never thought to try out for until the head cheerleader taunts her and tries everything she can to keep her from joining. Breanna has had several crushes, including Arnaz, one of her two best friends, who became an on-again/off-again love interest during the series, and in whom she did not show any romantic interest until the end of the first season. Breanna dated Josh, a handsome but dimwitted boy, who became part of a love triangle between Breanna, Josh, and Arnaz. Eventually, Breanna and Arnaz became a couple, only to break up when Josh and Arnaz' ex-girlfriend Ginger came up with a plan to break the two up, in which Breanna caught Ginger kissing Arnaz. During the third season, Arnaz and Breanna seemed to be on the way to getting back together until he revealed he was not a virgin one night when Breanna wanted to lose her virginity to Arnaz to stay popular with the cheerleaders (she did not go through with it after she found out). Then, Breanna decided to remain a virgin until she was ready to have sex, even if it was not until she was married. Breanna also kind of went out with a boy from her school, Nyghtmare, whose real name was Clayton. He was smart, but had a reputation as a bad boy. When Breanna found out, she said something that almost leads to a kiss. Flex saw them and prevented Breanna …

Read more on Wikipedia

The show's theme song "Living One on One", was written and performed by Shanice (wife of the show's star Flex Alexander) and Tonex with music composed by Jonathan Wolff and Becky Kneubuhl (who composed the scene-change music for the first season). The theme song was slightly shortened and remixed by Detail and Ray J for the show's fifth season. A truncated version of the theme, which had no lyrics except for the words "One on One", was used as a closing theme, heard only in syndicated airings.

The opening titles for the first three seasons featured the cast playing basketball on an outdoor court (though it also showed some characters doing other things at that same setting, e.g., Spirit checking out a boy who walks past her, and Arnaz playing the guitar only to be interrupted by a basketball bouncing in his direction that fell out of Duane's hand and he chases him). The fourth-season opening titles featured the cast in a dressing room getting ready for a night on the town, the end of the sequence featuring a pan shot of the cast in the mirror wearing different outfits from what they were wearing at the beginning of the pan shot. The opening sequences for the first four seasons included a closeup shot of Flex, Breanna, and Arnaz before cutting back to a shot of the entire cast. The final season's opening titles featured the main cast (Breanna, Arnaz, and new characters D-Mack, Sara, Cash, and Lisa) at different places at the Venice Beach Boardwalk, before meeting back at the beach house to take a picture together.

One on One began airing in syndication in September 2006. CBS Paramount Domestic Television also held the distribution rights to the series for the first year of its syndication run. CBS Media Ventures (formerly CBS Television Distribution) has been the series' distributor since September 2007.

In the United States, One on One was broadcast on various local television stations (mostly Fox, The WB, and UPN) affiliates from September 2006 to September 2009. The series was rerun on Noggin's teen block, The N, from October 2006 onward. It continued to air when The N was spun off into a 24-hour channel on December 31, 2007 and ran until October 2008. It rejoined the channel's lineup on September 14, 2009, and was moved to the successor channel TeenNick on September 28, 2009. It aired in two-hour blocks in the morning and late-night hours before being removed from TeenNick's lineup in 2013. The series began airing on BET in 2009, airing in two one-hour blocks (one weekday mornings and one in the afternoon), as well as Saturday broadcasts.

Formerly, Bounce TV and TV One both air the series. The series joined Netflix on October 15, 2020.

As of 2024, reruns of the series can be seen on Cleo TV and Dabl.