The young spanish played 34 times for the blues after joining from Barcelona in 2011.
The 23 year old spent two seasons on loan playing for German side Stuggart and Spanish Side Valencia,he was on the bench for Chelsea’s Uefa Champions League final against Bayern Munich in 2012,but miss out on the FA cup and 2013 Europa League victory.
Reed also said they are happy to bring Oriol to the club on a permanent basis after him being on their radar for a long time.
Once again, we are demonstrating our commitment to compete seriously on all fronts this season by continuing to build a squad with depth and quality.
“Oriol is a player we have had on the radar for some time – he was a prominent figure in the Chelsea team of 2011/12, when he made 24 appearances as a 20 year-old.
“Like Ryan Bertrand, we believe we have recruited another Chelsea player with an excellent pedigree but also with lots of potential to develop.
“He is another player who matches the Southampton profile – he is young and hungry, has been developed at a top academy and has already played at the highest level with plenty more to come.
“He will complement the qualities we already have among our squad and provide another level of competition for selection.”