Onana is expected to replace Douglas Luiz, who left Villa for Italian Serie A side Juventus earlier.
Aston Villa have signed Netherlands defender Ian Maatsen from Premier League rivals Chelsea
Jhon Duran struck twice in the final five minutes to salvage a 3-3 draw for Aston Villa against Liverpool but they fell short of guaranteeing a place in next season's Champions League on Monday.
Olympiakos will face Fiorentina in the Europa Conference League final after making history with their 2-0 win against Aston Villa in Thursday's semi-final second leg.
Aston Villa's bid to finish in the Premier League's top four was rocked by a 1-0 defeat at Brighton, while Chelsea thrashed West Ham 5-0 to keep alive their hopes of European qualification on Sunday.
Olympiakos showed no signs of nerves in their first ever European semi-final with Ayoub El Kaabi taking his tally for the season to 30 goals.
Fifth-placed Tottenham's third successive defeat leaves them seven points behind fourth-placed Aston Villa.
Aston Villa head into a first European semi-final for 42 years against Olympiakos on Thursday but have the experience of a proven winner at this stage of continental competition in manager Unai Emery.
Mauricio Pochettino claimed VAR has damaged the Premier League after Chelsea had a late goal controversially disallowed in their dramatic 2-2 draw at Aston Villa on Saturday.
Emiliano Martinez starred in a drama-filled penalty shootout as Aston Villa squeezed past Lille and into the Europa Conference League semi-finals.
Arsenal's Premier League title hopes suffered a massive blow as Aston Villa struck twice in the closing stages to boost their top four bid with a dramatic 2-0 win.
As the title race approaches its final stretch, leaders Arsenal face a potentially decisive reunion with their former boss Unai Emery.
Manchester City had not beaten a side in the Premier League's top five in six attempts this season prior to the visit of fourth-placed Villa to the Etihad.
Spurs have now come from behind to win in four of their last five home games and needed another second-half turnaround after Tahith Chong's strike put Luton into a shock lead after just three minutes.
Ajax and Villa struggled to create clear-cut chances and when the hosts did finally get one, Emiliano Martinez was equal to the task with a fine diving save to deny Kenneth Taylor.
Barcelona's 16-year-old starlet Lamine Yamal struck twice to rescue the struggling Catalans a 3-3 draw against Granada in La Liga on Sunday.
Scott McTominay kept alive Manchester United's hopes of finishing in the Premier League's top four as the midfielder's late goal clinched a vital 2-1 win against Aston Villa on Sunday.
Scott McTominay kept aliveManchesterUnited's hopes of finishing in the Premier League's top four as the midfielder's late goal clinched a vital 2-1 win against Aston Villa.