Oier joins Granada
The 24-year-old joined Barca's youth academy in 2007 as an 18-year-old, but was unable to make the full step up to senior level.
Oier made just two senior appearances in his time at Camp Nou, but will hope for more playing time at Granada.
The Andalusian club narrowly avoided relegation from La Liga last season, finishing just two points clear of the drop zone in 15th.
Oier will have a challenge on his hands if he is to displace regular goalkeeper Roberto, who made 32 appearances for the club last season.