Tarkowski joined League One club Oldham Athletic on a two-year scholarship in May 2009. He made his reserve-team debut in October 2009. A centre back,Tarkowski was called up to the first team for the first time for a league match versus Plymouth Argyle on 15 January 2011.His senior debut came on 22 January, coming on as an 80th-minute substitute for the injured Neal Trotman in a 2–1 home win over Brentford. Tarkowski made his first Latics start in a 1–0 defeat to Leyton Orient on 12 March and played the full 90 minutes.He was awarded a two-year professional contract by the club in May 2011. He made 9 appearances during the second half of the 2010-11 season.
Tarkowski had to wait until 10 December 2011 to make his first appearance of the 2011-12 season,when he came on as a 61st-minute substitute for Jean-Yves Mvoto in a 2–0 home defeat to Sheffield Wednesday. A half time substitute appearance in Oldham’s 2–1 Football League Trophy Northern Area final first leg defeat to Chesterfield on 30 January 2012 kicked off a run which saw Tarkowski included in almost all of Oldham’s first-team squads until the end of the season.Tarkowski crowned his run of appearances by scoring his first senior goal for the club on the final day of the season in a 2–1 win over Carlisle United.He made 18 appearances during the 2011–12 season and scored one goal.
Tarkowski playing for Oldham Athletic in 2012.Tarkowski began the 2012-13 season as a first-choice centre back alongside Jean-Yves Mvoto and started Oldham’s first six games of the season. He scored his first goal of the season in a 2–0 league win over Crewe Alexandra on 2 October 2012. Tarkowski played a part in Oldham’s run to the fifth round of the FA cup, playing in the first round win over Conference Premier team Kidderminster Harriersand starting in the home draw and away replay defeat to Premier League team in the fifth round. He scored his second goal of the season in a 2–1 league win over on 19 February 2013.Tarkowski made 26 appearances and scored two goals during the 2012–13 season.He signed a new two-year contract in the summer of 2013.
Tarkowski began the with a bang, scoring Oldham’s first goal in a 4–3 away league win over Stevenage on the opening day.He was picked by the Latics supporters as Oldham’s Player of the Month for August 2013.Now lining up at centre back alongside Tarkowski scored his second goal in a 2–1 defeat to on 9 September 2013.Tarkowski was an ever-present in Oldham’s runs to the Northern Area semi-final of the Football League Trophy and the third round of the FA Cup.He started the FA Cup third round tie against Premier League team at and scored an 82nd minute , which consigned the Latics to a 2–0 defeat.Tarkowski’s form saw him offered a new contract extending until the end of the 2016–17 season on 22 January 2014, but he departed 9 days later.During his time with the Latics, Tarkowski made 89 appearances and scored six goals