England have struggled to fill their left-wing berth over recent years, with numerous players staking a claim for the role, but Downing could be a long-term solution.
Warnock, meanwhile, will be hoping to fill the left-back void created by Ashley Cole's untimely injury, or at least be drafted into the fold as back-up.
O'Neill feels both men would not look out of place on the world stage and hopes they will be given an opportunity to prove their worth at international level.
"I've said this before, surely there must be places up for grabs that the England manager is not completely decided upon," O'Neill told the Birmingham Mail.
"Stephen has been hovering around the squad just recently and that's good news and Stewart, of course, has been out with injury, but I think Capello thought a lot of him beforehand anyway. All of those things will encourage them."
O'Neill has reserved special praise for wing wizard Downing, with the former Middlesbrough man having sparkled for Villa this term since shaking off a long-standing foot problem.
"I think there's a lot more to come from him," he said.
"Again, he's had no pre-season, he's just been thrust into it and played a lot of games in succession.
"He hasn't really had a break either. I've been really pleased with him. I think he's slotted in very nicely.
"He's got great delivery and I think he has demonstrated in periods of games here the ability to go past people to leave them for dead.
"I think that's the sort of thing that he can do and that will become more prevalent as he gets to grips with it all."
Aston Villa v Burnley 1st Goalscorer: Downing, S. 10/1