The 30-year-old centre-back made 95 appearances and scored four goals in three seasons at Tynecastle, playing 25 times last campaign as Hearts narrowly missed the Scottish Premiership title.
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He began his career with Colchester United, making 141 appearances, before 149 league games across four seasons with Peterborough United.
After his release this summer, he becomes Oxford United’s first signing since Aaron Ramsey was appointed head coach role.