Oliech started playing football for Dagoretti Santos in his earlier years, then Mathare United. While in his youth career, Oliech went on trial with Ligue 1 side Marseille, but Mathare United chairman Bob Munro blocked the move from happening.
Oliech turned professional in 2003 playing for Al-Arabi in Qatar until late 2005. In 2004, he was offered a chance to change his citizenship from Kenyan to Qatari. But he refused the offer.At age nineteen, Oliech was named by The Guardian newspaper as one of the world’s most promising young players in a list which included future stars such as Wayne Rooney and Robin Van Persie.
While at Qatar, Oliech played alongside Stefan Effenberg and Gabriel Batistuta during their time at Qatar. In 2005, Al-Arabi turned down a Sh100m offer from Monaco to sign Oliech. During the same year, Al-Arabi was told to pay US$15,000 compensation to his youth club by the Fifa’s Dispute Resolution Chamber (DRC), as the move to sign Oliech. His goalscoring form is well known for goal poacher.
In 2005, Oliech signed a four-year contract with the French Ligue 1 team Nantes, with the fee of $3.7 million (Ksh. 281.2 million). Upon the move in January, Oliech was given a number nine shirt. However, his youth club Mathare United claims Nantes owed them Sh14m, but the claims were dismissed.
Shortly after signing for Nantes, Oliech hadn’t made a great start in France after a knee operation.Then, thirty-days later, he was given all-clear by the doctors and returned to training. He made his debut, on 11 March 2006, when he came on for Julio Hernán Rossi, in the 66th minute, as Nante drew 1–1 with Lille.Three weeks later, on 1 April 2006, Oliech scored his first goal, in the round of 16 in the Coupe de France to send them to the quarter-final, in a 1–0 win over Dijon. and his second goal was followed up in the league match, in a 3–1 loss against Marseille. Later in the 2005–06 season, Oliech made nine appearance and scoring twice.
In the 2006–07 season, Oliech scored four times in thirty-two appearance and the season was proved to be difficult for him, as he soon lost his first team place Despite lack of first team opportunities and linked with a Premier League club, Oliech says he is happy at the club. However, FC Nantes were relegated in 2007 after finishing bottom of Ligue 1.
In the summer transfer window, Oliech was signed on loan by Auxerre who were in need of goals after a bad start to their season. After two months absent, Oliech finally made his debut, in a 5–3 win over Lorient. After making ten appearance so far in the 2007–08 season, Auxerre signed him on a permanent basis from Nantes in January and keeping him until 2011. Later in the 2007/2008 season, Oliech scored three goals in 24 games, including a brace against Rennes.
He endured what were considered below-average seasons by most in the 2008–09 (scoring three goal in thirty-three appearance in all competitions) and 2009–10 season (scoring four goal in thirty-seven appearance in all competitions), as his form dropped badly, due to his making several match deciding errors and misjudgements. Then, in the 2010–11 season, Oliech received more confidence by Manager Jean Fernandez, who gave him more playing time. On 25 September 2010, Oliech scored twice in a 2–2 draw against Rennes. After featuring twice in the Champions League so far, Oliech was sent-off after receiving a second yellow card, in a 2–1 loss against Ajax in Matchday Three in the Champions League Group-Stage. Though Auxerre did badly in the Champions League Group-Stage, finishing last, with one point, Oliech featured five games. On 5 January 2011, he signed a new two-year contract, which he will remain until at least the summer of 2013. At the end of the 2010–11 season, Oliech scored four times in forty-three appearance.
In 2011–12 season changed when the club appointed Laurent Fournier as their new manager. On 25 September 2011, Oliech scored his first ever hat-trick for Auxerre in a 4–1 win over Sochaux. The hat-trick was accomplished within the span of 16 minutes. Soon after in early-December, Oliech scored two goals in two consecutive matches, against PSG and Nice. Though later in the season, his form dropped badly, due to making several match deciding errors and misjudgements, having his goalscoring form had gone less and it couldn’t get worse for the club, as they are relegated to Ligue 2, but Oliech was the club’s top scorer.
At the end of the 2011–12 season, Oliech have since attracted interests from clubs around Europe and France and expected to leave the club. Oliech was close of joining Turkish side Kasımpaşa for worth two million and contract detail of three years. But unfortunately, the move couldn’t be agreed and he made a return to Auxerre. In the 2012–13 season, Oliech would score three goals in seventeen league appearance in the first half of the season.
In the January transfer window, Oliech announced he wanted to leave Auxerre, ending a six-year association with the club on 15 January 2013, so he could take on a fresh challenge.On 22 January 2013, Oliech was unveiled by Ajaccio of Ligue 1 after completing a move from Ligue 2 side Auxerre and signing a two and a half-year contract. He was handed the number 29 jersey and went on to score on his debut against Evian Thonon Gaillard in a 1–1 draw, which he was delighted to score on his debut.
On 12 February 2015, Oliech terminated his contract with AC Ajaccio, which was to run until June 2015 on mutual consent. The club then announced that Oliech would join UAE Arabian Gulf League side Al-Nasr. However, on 21 April 2015, it was confirmed that Oliech had joined Dubai CSC in the UAE Division 1 league and not Al-Nasr.
On 2 January 2019, 33-year-old Dennis Oliech agreed a two-year contract with Kenyan giants Gor Mahia, becoming the highest paid footballer in the country at a fee of Kshs.350,000.
Dennis Oliech International career
His first international match was in 2002, when Kenya lost to Nigeria 3–0.
He scored five goals at the 2002 CECAFA Cup, leading Kenya to the title as well as getting the top scorer award.
In 2005, Oliech made controversy when he refused to join the national team squad after officials from the Kenya Football Federation (KFF) told him was owed them US$1,700. His mother defended him, believing he has a right to demand for his money from the Kenya Football Federation.Days before that, Oliech injured his knee ahead of a 2006 World Cup qualifier, but later revealed he quickly recovered from the minor injury. Eight days later, Oliech apologised for his behaviour over the entire situation.
On 24 March 2016, Oliech announced his retirement from international football
Oliech’s older brother, Nixon Onywanda, is the coach of the Kenyan side Dagoretti Santos (for which Oliech once played).Oliech has a son with his girlfriend and drives a porche car and lives in an expansive house.In June 2011, Oliech purchased himself a new car, the Chrysler 300C SRT-8 series.
In late 2005, a warrant of arrest was issued against Oliech after he failed to appear in court to answer to assault charges raised against him.
In late-May 2006, Oliech was admitted to hospital after a disagreement, which led to a brawl and left him with face injuries. This just came after being in hospital following his injury in the exhibition match in Maringo during a local tournament.