[4][5] On 31 August 2018, Arnór signed a five-year contract with the Russian Premier League club CSKA Moscow.

[9] Four days later, he scored his first Russian Premier League goal in a 2–0 victory over Zenit Saint Petersburg.

[citation needed] On 30 July 2021, he joined Italian Serie A club Venezia on loan for the 2021–22 season.

[12] On 3 July 2022, Arnór suspended his contract with CSKA Moscow for the 2022–23 season, having taken advantage of the FIFA ruling relating to the Russian invasion of Ukraine.

[16] Arnór made his senior debut for Iceland on 15 November 2018 in a 2018–19 UEFA Nations League A game against Belgium, as a starter.