Omigawa made his professional mixed martial arts debut in 2005 against future UFC veteran Aaron Riley at PRIDE Bushido 7. Omigawa and Riley traded punches before Omigawa was knocked out by a head kick six minutes into the first round. Omigawa then fought for the British Cage Rage organization against future two-time K-1 HERO’S Middleweight Champion Gesias Calvacante. He again lost this fight by knockout, this time from an overhand right.

The 12-8-1 Omigawa re-signed with the UFC in the Featherweight division and he faced then-undefeated Chad Mendes on February 5, 2011 at UFC 126. He lost the fight via unanimous decision.

Omigawa was defeated by Darren Elkins via unanimous decision on June 11, 2011 at UFC 131. The decision was considered controversial, and despite the loss, UFC president Dana White stated on his Twitter account that both fighters would be paid a win bonus.

Omigawa faced British fighter Jason Young on November 5, 2011 at UFC 138 and won the fight via unanimous decision.

Omigawa next faced former Jungle Fight Lightweight Champion Yuri Alcântara on January 14, 2012 at UFC 142. Omigawa lost the fight via unanimous decision.

Omigawa fought fellow judoka Manny Gamburyan on August 4, 2012 at UFC on FOX 4. He lost via unanimous decision. After back-to-back losses in the UFC, Omigawa was released from the promotion yet again.

In May 2013, it was announced that Omigawa had signed with Korea’s ROAD FC and would debut against fellow judo practitioner, Bae Young Kwon at Road FC 12. After a back and forth fight, Omigawa lost a controversial decision.

Bringing his boxing and judo skills to the kickboxing/grappling hybrid sport of shootboxing, Omigawa entered the 2013 –65 kg S-Cup held at Shoot Boxing Battle Summit Ground Zero Tokyo 2013 in Tokyo, Japan on November 15, 2013. After defeating Yuki Hiroshi Kimaya by unanimous decision in the quarter-finals, he then faced fellow mixed martial artist “Wicky” Akiyo Nishiura in the semis. The fight was ruled a majority draw after the regulation three rounds and so went to an extension round to produce a winner, after which Omigawa was given the nod by all three judges. In the tournament final, he fell victim to a flying knee from Hiroaki Suzuki, losing by knockout in round two.