Omielańczuk made his professional debut in 2009, competing primarily in regional promotions across the Eastern Europe, including KSW, where he compiled a record of 15 - 3 before signing with the UFC in the spring of 2013.
Omielańczuk made his promotional debut against fellow newcomer Nandor Guelmino on September 21, 2013 at UFC 165. After a back and forth first two rounds, Omielańczuk won the fight via knockout in the third round. Omielańczuk faced Jared Rosholt on April 19, 2014 at UFC Fight Night 39. Rosholt defeated Omielańczuk via unanimous decision. Omielańczuk was expected to face Soa Palelei on November 8, 2014 at UFC Fight Night 55. However, Omielańczuk was forced to pull out of bout with a broken thumb and was replaced by Walt Harris. Omielańczuk faced Anthony Hamilton on April 11, 2015 at UFC Fight Night 64. Omielańczuk lost the fight by unanimous decision. Omielańczuk was expected to face Konstantin Erokhin on July 18, 2015 at UFC Fight Night 72. However, Erokhin pulled out of the fight in late June citing an injury and was replaced by promotional newcomer Chris de la Rocha. He won the fight via TKO in the first round. Omielańczuk faced Jarjis Danho on February 27, 2016 at UFC Fight Night 84. He won the fight via technical majority decision after Danho was hit with an inadvertent groin strike in the third round that rendered him unable to continue. Omielańczuk next faced Alexey Oleynik on July 13, 2016 at UFC Fight Night 91. He won the fight via majority decision. Omielańczuk faced Stefan Struve on October 8, 2016 at UFC 204. He lost the back and forth fight via submission in the second round. Omielańczuk faced Timothy Johnson on March 18, 2017 at UFC Fight Night 107. He lost the back and forth fight by split decision. Omielańczuk faced Curtis Blaydes on July 8, 2017 at UFC 213. He lost the fight by unanimous decision.
Post-UFC
After his third consecutive loss, Omielanczuk was released from the UFC. He later signed with Absolute Championship Berkut, competing against American Bobby Brents at ACB 83: Baku in 2018. He won via first-round submission. After picking up a unanimous decision win in the regional kickboxing circuit, Omielanczuk returned to MMA to face Amir Aliakbari at ACB 89: Krasnodar in September of 2018. He was defeated via unanimous decision. Since the loss to Aliakbari, Omielanczuk has won his last three fights pushing him to 4-1 after leaving the UFC.
