Maskaev, a former mine worker, began his career in the 1980s as an amateur boxer in the Soviet Union, where he held a victory over later WBC world champion Vitali Klitschko. He was the champion of the Soviet Army and later the national cup winner. Representing Uzbekistan, he won a gold medal at the 1994 Asian Games. His professional boxing career began in 1993.
Oleg is known for his powerful right-hand punch: he has knocked out former WBO heavyweight challenger Derrick Jefferson, contender Alex Stewart, and twice knocked out former WBC heavyweight champion Hasim Rahman. However, he is also noted for a weak chin that was evident in his knockout losses to contenders Oliver McCall, David Tua, Kirk Johnson, Lance Whitaker, Corey Sanders and journeyman Nagy Aguilera.
His most famous victories have been his two wins over Hasim Rahman. Maskaev knocked Rahman off balance, sending his rival out of the ring in the 8th round on November 6, 1999. This fight is also notable for the famous "chair incident", in which noted referee, Steve Smoger, who was backup referee for this fight, was struck in the head by a chair thrown by a fan who was later arrested.
He later fought Rahman again and defeated him for the WBC heavyweight title by knocking him out in the 12th round on August 12, 2006. His second win over Rahman was preceded by a streak of victories that helped rejuvenate Maskaev's career and earned him the "Comeback fighter of the Year" award in The Ring magazine in 2006.
Maskaev won his first title defense against Peter Okhello on December 10, 2006 by unanimous decision.
Maskaev fought against Samuel Peter for the WBC Heavyweight title on March 8, 2008 and was doing well in the early rounds, despite Peter's repeated rabbit punching and the referee's warnings. In the 5th round Maskaev landed some of his best shots on his opponents chin, but to no avail: Peter was seemingly unaffected, visibly shaking Oleg. He lost 5 of the 6 rounds, with Peter winning by TKO with 3 seconds remaining in the sixth round.
In late 2008 he beat opponent Robert Hawkins by UD after 10 rounds and then defeated but unheralded Rich Boruff on March 14, 2009 via first-round KO, giving him mandatory contender status to the WBC Heavyweight Championship currently held by Vitali Klitschko. He fought Nagy Aguilera on December 11 in Sacramento, California at the Memorial Auditorium in a tune up fight, but was knocked out in the first round. Getting stunned with an overhand right-left hook combo and knocked down soon after, another barrage by Aquilera hit Maskaev hard knocking him out. Maskaev did not land a punch in the fight, and has since stated that he may retire following the loss, however he returned to the ring 3 years after the loss, to knock out Owen Beck. In 2013, he decisioned Jason Gavern in a 10-round fight.
Maskaev was born in Taraz to Mordvin parents. He has lived in the U.S. since 1999 with his wife, Svetlana, and four daughters. He acquired US citizenship in 2004. He currently resides in West Sacramento, California. Before his second encounter with Hasim Rahman in 2006, he said "I would say I'm a proud Russian-American. So right now, I'm a citizen of America, of [the] United States... Whoever is going to win is going to be American." In the run-up to his title defense against Peter Okhello, he stated regarding his citizenship: "Russian. I don't want to talk about that anymore. I will walk to the ring under the Russian flag and Russian anthem as I'm now a Russian citizen." Russian president Vladimir Putin granted him Russian citizenship on December 9, 2006.