by playing in all of the club's end of season matches, including both of the play-off semi-final matches against Dover Athletic, Sinclair missed the National League play-off final defeat to Grimsby Town at Wembley Stadium due to injury. At the end of the season, he agreed a new two-year contract to remain at the club until the summer of 2018. Having been a regular starter during the first half of the 2016–17 campaign, injury meant that Sinclair played fewer matches during the second part of the season. He played just once in the final two months of the season, missing all of the club's play-off matches as Forest Green earned promotion via the National League play-offs at the third time attempt. He made 22 appearances during the campaign. Oxford City Having not made any appearances for Forest Green in the opening weeks of the 2017–18 season, Sinclair left the club to join National League South club Oxford City on a free transfer on 25 August 2017, despite interest from National League clubs Barrow and Wrexham. He made his debut for the club a day after signing, scoring the first goal of the match in a 1–1 draw at home with Welling United. Sinclair was a part of the squad that reached the FA Cup second round for the second time in the club's history, playing the whole match in a 1–0 away win at League Two club Colchester United on 4 November 2017. He also scored in Oxford City's 3–2 loss to Notts County at Meadow Lane in the following round on 2 December 2017. Sinclair scored three times in five matches during December, and was subsequently named as the National League South Player of the Month. Sinclair played in the final as Oxford City won the Oxfordshire Senior Cup courtesy of a 5–3 victory over Kidlington on 1 May 2018. He played regularly throughout the remainder of the season, making 41 appearances in all competitions, scoring 11 goals. He remained at Oxford City for the start of the 2018–19 season, scoring his first goal of the season in a 1–0 win over eventual champions Torquay United on 14 August 2018. He scored four times in 12 games in the opening three months of the season, including twice in a 5–0 FA Cup victory against Hemel Hempstead Town on 23 October 2018. He left the club by mutual consent on