All four will be out of contract on June 30 and able to leave on free transfers.
"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," Liverpool said.
"The four all played their part in Jurgen Klopp's side winning the Champions League, UEFA Super Cup and FIFA Club World Cup in 2019, before becoming the first Liverpool team to clinch the league title for 30 years as they racked up 99 points in 2019-20.
"Two more trophies were added to the collection amid a tilt at a quadruple last term, with success in the Carabao Cup and Emirates FA Cup at Wembley."
Liverpool also announced that a special tribute would be made to the four during the club's final home game of the season, against Aston Villa on Saturday.
"With the visit of Aston Villa on Saturday being the Reds' final home fixture of the campaign, special acknowledgements will be paid to the quartet at Anfield, with further tributes to follow across the club's digital platforms after the end of the season as we wish Roberto, Naby, James, Alex and their families the very best for the future," the club added.
Sources told ESPN Brasil in March that Firmino had decided not to renew his contract, despite Klopp expressing his desire for the Brazilian forward to extend his deal earlier in the campaign.
Firmino, who joined Liverpool from German outfit Hoffenheim in a £29 million deal in 2015, is said to want a new challenge.
The departures of midfield trio Keita, Milner and Oxlade-Chamberlain were widely anticipated ahead of what is expected to be a busy summer in the transfer market for the Merseyside club.
Keita joined Liverpool from RB Leipzig for a then club-record fee of £52.75m in July 2018 but amid a series of fitness issues has struggled to live up to expectations at Anfield, making 129 appearances in five seasons.
Injuries also limited the impact of Oxlade-Chamberlain since he arrived from Arsenal for £35m in August 2017.
Milner, 37, has been at Liverpool for the entirety of Klopp's tenure and the Liverpool boss recently paid tribute to the influence of the former England international.
"[Milner] knows how highly I value him," Klopp said earlier this month.
"He is an incredible part of this team and I would love to work together with him even longer.
"I said a couple of times nothing positive that happened in the last seven and a half years would have happened without James Milner."
Liverpool have endured a disappointing season, though a run of seven successive wins has given them an outside chance of qualifying for next season's Champions League. With two games to play, they sit fifth, a point behind Manchester United and Newcastle United though having played a game more.