Classic:
* O.C., The (2003-2007): Teen drama set in Orange County, California.
* Ozark (2017-2022): Crime drama about a financial advisor who moves his family to the Missouri Ozarks to launder money.
* One Tree Hill (2003-2012): Teen drama about two half-brothers who are rivals on the basketball court.
* Orange is the New Black (2013-2019): Comedy-drama about a woman who is sent to prison for a crime she committed years ago.
* Outlander (2014-present): Historical romance drama about a World War II nurse who is transported back in time to 18th-century Scotland.
* Only Fools and Horses (1981-2003): British sitcom about two brothers who run a variety of businesses, often with disastrous results.
Modern:
* Only Murders in the Building (2021-present): Comedy-mystery about three strangers who share a love for true crime and find themselves investigating a real murder in their apartment building.
* Outer Banks (2020-present): Teen drama about a group of friends who live on the Outer Banks of North Carolina and are searching for treasure.
* Obi-Wan Kenobi (2022): Star Wars series following the titular character's adventures between Episodes III and IV.
* Overlord (2018-2019): Fantasy horror series set in a dystopian future.
* Our Flag Means Death (2022-present): Historical comedy-drama about a wealthy aristocrat who decides to become a pirate.
* On My Block (2018-2021): Teen comedy-drama about a group of friends growing up in a rough neighborhood.
Other Notable Shows:
* Orphan Black (2013-2017): Science fiction thriller about a woman who discovers she is a clone.
* One Day at a Time (2017-2020): Comedy about a Cuban-American family in Los Angeles.
* Ozark (2017-2022): Crime drama about a financial advisor who moves his family to the Missouri Ozarks to launder money.
* Once Upon a Time (2011-2018): Fantasy drama about fairy tales characters coming to life in a small town in Maine.
* Office, The (2005-2013): Mockumentary sitcom about the daily lives of office workers in Scranton, Pennsylvania.
* Ocean's Eleven (2001-2004): Crime drama about a group of thieves who plan to rob three Las Vegas casinos.
This is just a small sample of the many TV shows that start with the letter "O". There are many other great shows to explore, depending on your taste.