Nong-O Sit Or was born as Apichet Khotanan in Sakon Nakhon Province in the Northeastern region of Thailand. He had his first fight at the age of 9. He became interested in Muay Thai when he saw one of his neighbors training. Nong-O was invited to train, and after a month of training he had his first fight.
2008
On August 3, 2008, Nong-O fought against Kōji Okuyama at Keio Plaza hotel in Hachiōji, Tokyo, and he won by KO with left elbow strike during 3rd round. December 9, 2008, he fought against Wutidet Lukprabath at Lumpinee Stadium in the memorial event of Lumpinee Stadium establishment. He won by decision after five rounds.
2009
On January 18, 2009, Nong-O fought against Tomoaki Suehiro at Yoyogi National Gymnasium for the Muay Thai event "Muay Lok". He won by TKO when the referee stopped the contest after he knocked Suehiro down twice with a right hook and left hook during first round. After the contest, he was given a bunch of flowers by Shinya Aoki. Nong-O was going to fight against Shunta Itō in the beginning, but he was replaced because his legs had not healed enough. On November 8, 2009, he fought against Trijak Sitjomtrai in Japan. Trijak was the current champion at featherweight sanctioned by the Professional Boxing Association of Thailand, but Nong-O won by majority decision after five rounds. On December 8, Nong-O fought against Petboonchu F.A. Group for his Lumpinee Stadium title at Super featherweight. Nong-O won by decision and became the new champion.
2010
On January 17, 2010, Nong-O participated in the tournament "Yod Muay Champions Cup 60kg" in Japan. He fought against Sindam Kietmukao in the first match. After five rounds, the bout was declared a draw. Nong-O lost by the split decision after extra round. On December 9, he challenged Saenchai's titles of Lumpinee Stadium and WMC, but he lost by decision.
2011
On January 19, 2011, Nong-O Sit Or was named 2010 boxer of the year at the Society of Friends of Sportswriters Awards Night in Bangkok.
2012
Nong-O started the year with a win over Singdam in February and followed it up the following month with another win over F16 Rajanon. He then lost a very close fight to Singdam in May, which many thought Nong-O had done enough to win, but beat Petboonchu FA Group in June. On 31 July he fought Singdam in the main event at Lumpinee Stadium and was defeated by unanimous decision. He faced Petboonchu F.A. Group for the seventh time on October 12, 2012 at Rajadamnern Stadium. He TKO'd Mongkolchai Phetsupaphan in five on February 7, 2013. He beat Singdam Kiatmuu9 on points to win the Lumpinee lightweight belt on June 7, 2013.
Return after hiatus to ONE Championship
2018
Following a competitive hiatus in which he moved to Singapore to coach at Evolve, Nong-O returned in 2018 when ONE Championship unveiled their new striking format: the ONE Super Series. Nong-O debuted in April 20 at and defeated Fabio Pinca via unanimous decision. He returned to his Thai homeland in October at, and defeated Mehdi Zatout via unanimous decision.
2019: ONE Muay Thai Bantamweight World Champion
On February 16, 2019, Nong-O defeated Han Zihao by unanimous decision to become the inaugural ONE Bantamweight Muay Thai World Champion at in Bangkok. On May 10, 2019, he would defend his ONE Bantamweight Muay Thai World Championship against Hiroaki Suzuki at, winning by unanimous decision and retaining his title. He made his second title defense against Brice Delval at in Ho Chi Minh City on September 6, 2019, featuring ONE Championship's first event in Vietnam and first all-striking card, where he successfully retained the ONE Bantamweight Muay Thai World Championship via a closely contested split decision win. Nong-O is scheduled to make his third title defense against Saemapetch Fairtex at ONE Championship: Edge Of Greatness on November 22, 2019. Nong-O successfully defended his ONE Bantamweight Muay Thai World Championship against Saemapetch Fairtex, winning by fourth-round knockout in a fight where he knocked down Saemapetch twice in the second round.
