[1] In December 2015, Óttar agreed to join Sparta Rotterdam on loan for the remainder of the 2015–16 Eerste Divisie season.
[3] On 25 November 2016, Óttar signed a three-year contract with Norwegian Tippeligaen side Molde FK.
[4] In February 2018, Óttar agreed to join Trelleborgs FF on loan for the upcoming 2018 Allsvenskan season.
[11] On 31 January 2023, Karlsson was loaned to Serie C club Virtus Francavilla until the end of the season.
[17] In 2022, he was called up to the team for international friendlies against South Korea and Saudi Arabia.