The youngster has signed a four-year deal and he will join the Blues for a 5million euros transfer fee.
Barcelona have the option to buy-back the midfielder for 10million euros after one year and 15million euros after two.
Chelsea boss Andre Villas-Boas said: “It is a part of the squad in which we are struggling with numbers because of Michael Essien’s injury and Oriol is a player with a bright future. He still has to evolve as a player.
“The Barcelona B team had a magnificent year in the Spanish second division, they finished third.
“Oriol had an injury during the season but he came back late on and was in time to play for the first team.
“So he is one of the most promising young players in that position.”