O'Hara was pictured entering Manchester Airport alongside Anthony Martial after Mourinho elected to bring the 20-year-old with him for United's Europa League last 16 first leg clash.
The shot stopper also made the trip to France when the Red Devils beat St Etienne 1-0 but he was left out of the matchday squad as Mourinho opted to start Sergio Romero and had David de Gea on the bench.
Kieran O'Hara (right) makes his way into Manchester Airport with Anthony Martial
O'Hara, pictured ahead of United's match against Northampton, will be hoping to impress
Manchester United goalkeeper David de Gea remains the club's No 1 but could be rested
O'Hara is again expected to be left out of United's matchday squad as Mourinho will want to call on one of his experienced keepers for a European clash which could lead to their only hope of playing Champions League football next season.
However it is a sign that Mourinho wants O'Hara to be in and around the first-team picture, learning from two international stars in De Gea and Romero.
O'Hara has been afforded the opportunity as both Joel Pereira and Dean Henderson are currently sidelined through injury while Sam Johnstone is on loan at Aston Villa.
So who is O'Hara? He joined Manchester United in July 2012 and signed his first professional deal with the Premier League giants two years later.
He found reserve-team football hard to come by due to Pereira's progression so he has had to bide his team out on loan.
O'Hara has been shipped out on a temporary basis five times with the keeper spending time at Trafford, AFC Fylde, Stockport and Morecambe on two occasions.
His latest spell at Morecambe saw him join the Shrimps on January 12, 2016 until the end of the season.
Sergio Romero (right) will deputise for De Gea if the Spaniard is rotated ahead of Chelsea
O'Hara has moved up the pecking order following Sam Johnstone's loan move to Aston Villa
With Pereira and Henderson currently out of the reckoning, O'Hara is hoping to impress Mourinho and his coaching staff while training with the likes of De Gea, Romero and goalkeeper coach Emilio Alvarez.
If he does well, O'Hara could travel away with United for the club's annual pre-season tour.
O'Hara has decided to represent the Republic of Ireland rather than England so he will not be filling Joe Hart's boots anytime soon.
He told manutd.com about his: 'My name is a bit of a giveaway.
'My grandparents on my dad's side were both born in Galway. The call came towards the end of last season when the Under 21 coach Noel King gave the Morecambe manager [Jim Bentley] a ring to ask if I'd be interested in getting involved.
'I was more than happy because it was an international call-up. When I was about 17, I had England Schools games against Scotland, Republic of Ireland and Wales but my dad's happy now with it being his side of the family and my mum is proud as well.
'It's a great experience being involved in international football and more games, a different type of football completely, so I hope I can grab it with both hands.'
Highly-rated Portuguese keeper Joel Pereira has been sidelined through injury since February