A midfielder, he started his career with IF Elfsborg in 2010 and went on to represent PSV, Palermo, Genoa, Panathinaikos, and Dynamo Moscow before retiring at AaB in 2021.

A full international between 2012 and 2020, he won 28 caps for the Sweden national team and represented his country at UEFA Euro 2016 and the 2018 FIFA World Cup.

He participated in many national team camps as a young player, and was predicted a great career for his mature style of play.

He made his first team debut in Elfsborg's 2–2 draw away at IF Brommapojkarna on 26 September 2010, and scored his first goal against Syrianska FC in a 2–1 home victory on 27 June 2011.

He also won the prestigious Månadens Tipselitspelare prize with the jury's citation: "Oscar Hiljemark has been endowed with a massive winning attitude that he has used to the competitive environment for the optimum development.

He was linked with Italian giants Milan and Napoli for a possible transfer, as well as Premier League teams Norwich City, Southampton, Watford and West Ham.

On 26 January 2017, Palermo have officially announced that Oscar Hiljemark has joined Genoa, on loan with an obligation to buy.

On 31 August 2017, Genoa midfielder Oscar Hiljemark arrived in Athens to complete his loan move to Panathinaikos, with the latter retaining the option to purchase the player at the end of the 2017–18 season.

[8] On 30 January 2018, Hiljemark's loan spell from Genoa was terminated by mutual consent, mainly because of the player's unwillingness to continue under the club's financial decline and interruption of funding.

In May 2018 he was named in Sweden's 23-man squad for the 2018 FIFA World Cup in Russia, and played in the group stage games against South Korea and Mexico.