The four players were part of the squad that won the Champions League in 2019 and helped the club win their first English league title in 30 years in the 2019/20 season.
"All four players will depart with our gratitude and appreciation for the contribution they have made," Liverpool said in a statement.
"The quartet will each embark on new chapters in their careers following the conclusion of the 2022-23 season, having helped the Reds lift a host of the game's biggest honours during their spells at Anfield.
"We wish Roberto, Naby, James, Alex and their families the very best for the future."
Firmino, who joined Liverpool in 2015, played alongside Mohamed Salah and Sadio Mane as part of a prolific front three under manager Jurgen Klopp. He has scored 80 goals in 254 league games and gained cult status among the fans.
However, he has not played since mid-April due to a muscle injury and is battling to be fit for what will be an emotional farewell against Aston Villa on Saturday in their final game at Anfield this season.
Milner also joined in 2015 from Manchester City and has made 330 appearances for the club.
The veteran has been a reliable servant but his game-time has been reduced in recent seasons with Klopp often using the midfielder off the bench.
Keita, who joined from RB Leipzig for a fee in excess of 50 million pounds, has 11 goals in 129 games since moving to Anfield in 2018. He has played only 12 times this season, however, after struggling with injuries.
Oxlade-Chamberlain has also struggled for playing time this season, making just 13 appearances. He joined in 2017 from Arsenal and has played 146 times for the club, scoring 18 goals.
Liverpool are fifth in the standings with 65 points, one point behind fourth-placed Manchester United, who have a game in hand. After playing Villa, they end the season with a trip to Southampton on May 28th.
