O'Neill told his squad that the November 2013 clashes with Latvia and Poland allowed individuals the chance to work their way into his future plans.
"These are very, very important," the Derry native said. "If I was going into a club for the first time, the first time I was meeting the players was in pre-season, then pre-season became very important. They became big games because I have to make judgements on players."
Friendlies have often been seen as a distraction from domestic club football, but throughout his tenure as Ireland boss, O'Neill has treated friendlies with respect, using them to test out players and assess his options.
Just two months ago, Blackburn Rovers defender Shane Duffy played his way into O'Neill's Euro 2016 plans with a dominant display against Switzerland, while the following friendly with Slovakia ruined Paul McShane and Rob Elliot's chances of going to France. Elliot will miss out through injury, but McShane's dismal performance ended his hopes of playing in the summer.
Friday's match with the Netherlands in Dublin and Tuesday's Turner's Cross encounter with Belarus will offer other fringe players the opportunity to stake their claim for Euro 2016.
Much of O'Neill's squad is already decided, with key players like Seamus Coleman, James McCarthy, and Jon Walters guaranteed to travel if fit. However, the 64-year-old revealed that 10 per cent of his panel is still up in the air.
"I won’t announce the squad but by the weekend 90 per cent will already know if they are travelling," O'Neill told reporters.
"There are a couple of players who haven’t played an awful lot in central midfield and that might be an area where we’re having a look and waiting until the second game. If a lot of the players had played the amount of football I’d have liked then I might have made my mind up early."
Central midfield is where O'Neill has the most selection headaches. McCarthy will not feature in either game through injury, but is a shoo-in for a space on the plane alongside Glenn Whelan, Jeff Hendrick, and Wes Hoolahan.
Who else makes the squad is anyone's guess. Harry Arter has been guaranteed gametime by the Ireland boss, but has just one cap to date, while Stephen Quinn and David Meyler have failed to set the world alight domestically in the Championship.
Darron Gibson reclaimed his place in the Everton team late in the season and could be an outside bet to make the squad, but with competition so fierce, the two friendlies this weekend will allow these fringe players to make a final case for inclusion.
The deadline for squad submission is 23:00 on Tuesday, and it would not be any surprise if O'Neill waited right until a minute before to make his final decision. However, a few good performances over the next few days could help force his hand.
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