The Dons entertain Bosnia and Herzegovina cup winners Siroki Brijeg.
And they are still seeking a replacement for Ash Taylor, the central defender who joined Northampton Town.
"Both Mark Reynolds and Anthony will see it as a real opportunity to get ahead of others," said McInnes.
Aberdeen had space for one "wild card" signing ahead of the first leg of the second qualifying round tie at Pittodrie, but that has been confirmed as winger Gary Mackay-Steven from Celtic.
O'Connor arrived from Burton Albion last summer and McInnes admits that the 24-year-old "was possibly disappointed" that many of his 40 appearances were not in his favoured position in central defence.
"It is clear I've been trying to bring in another defender because we are light in those areas," said the manager.
"But I'm confident we've got the experience and the understanding between the back four that we've got here.
"We will be signing another defender, but Shay Logan and Andrew Considine have been my full-backs for the last few years now and I'm comfortable we've got the experience, ability and understanding there to stop their threat."
Logan, who signed after leaving Brentford in 2014, this week agreed a three-year contract extension, with McInnes revealing that the £1.3m received from Celtic for winger Jonny Hayes helped smooth the negotiations with the right-back and goalkeeper Joe Lewis.
"We said when we lost Jonny it was important that we used the money wisely and important to try to reinvest into the squad and to identify resigning players currently with us," said the manager.
"By and large, I've been really pleased with what the new players have gave us and delighted to get Shay and Joe Lewis signed for longer."
While Republic of Ireland winger Hayes had gone in the other direction and Northern Ireland forward Niall McGinn had also opted to leave, midfielder Greg Tansey has arrived from Inverness Caledonian Thistle, while strikers Nicky Maynard and Greg Stewart have been recruited from MK Dons and Birmingham City respectively.
Celtic midfielder Ryan Christie has also stuck around for a second season on loan and Logan, speaking before the arrival of Mackay-Steven was confirmed, revealed he was also pleased with the way the squad is shaping up.
"I am more than happy with the games we've had and we go into this game confident and hopefully progress to the next round," added the 29-year-old while admitting he does not know very much about the Bosnian visitors.
"It is a division that not many people probably watch, but we've had a few scouts at games and watched a few videos, but we won't know more until we play them.
"Hopefully we can get a good result and hopefully know more about them going to their ground.
"We have to give them the utmost respect and go into this in a professional manner.
"They are probably going to be underdogs, but sometimes underdogs win."
Aberdeen manager Derek McInnes has backed Anthony O'Connor to step up to the plate after failing to add a new defender before the start of their Europa League campaign.