Northern Ireland’s match in Cardiff comes less than 72 hours before Oxford host Hull City on Friday when, under normal circumstances, on-loan Spurs forward Donley and defender Brown would be certain starters.
Spencer has made 26 appearances for Oxford this season, and McDonnell has played 10 times since joining from Nottingham Forest in January.
Northern Ireland missed out on World Cup qualification after they were beaten 2-0 by Italy in Bergamo in the play-off semi-finals last Thursday.
Donley, who started in that game, scored the winner when Oxford beat Blackburn 1-0 at the Kassam Stadium earlier this month.
“I think 80% of my squad play in the EFL so I am aware of that situation,” O’Neill said.
“We’re mindful of the situation for the clubs, of course, but when the clubs signed these players they knew they were international players.
“We’re not going to be reckless with the players or anything like that. But we have to look after ourselves as a group of players, so the most important thing is that the players just go out and play the game.
“They will be fine. The lads who play in the EFL they play a lot of football and they are used to playing regular football. They have a lot of resilience so I’m sure they’ll get through the game fine.”
O’Neill has been in charge at Ewood Park since February, when he replaced Valerien Ismael.
The 56-year-old had a similar agreement – combining duties with club and country – with Stoke City when his first spell as Northern Ireland manager came to a close in 2020.
However, the Covid-19 pandemic delayed the Euro 2020 play-offs and O’Neill left his role before those games took place.
After being sacked by the Potters in August 2022, he returned to Northern Ireland, signing a contract that runs until after the Euro 2028 campaign.