The 26-year-old Swede joins the League One club on a two-year deal, with the option of a one-year extension.
Former Malmo and Hacken player Pekalski is Oxford United's fourth overseas signing since manager Pep Clotet was appointed in July.
"I'm happy to be here and it's been a pretty natural decision to make the move," he told BBC Radio Oxford.
"After 11 years of playing in Sweden, I was looking for something that would be a challenge and a bit of an adventure.
"I've worked with Pep before (at Malmo), he's a brilliant coach and I've heard a lot about Oxford's ambition, which I think is really interesting."
Pekalski has made 15 appearances for Halmstads in the Swedish top division this season after joining the club in March.
Find all the latest football transfers on our dedicated page.
Oxford United have signed midfielder Ivo Pekalski from Swedish side Halmstads for an undisclosed fee.