Long came off the bench for the Republic of Ireland on Thursday night, and netted the winner in their shock 1-0 victory over world champions Germany.
Koeman has started Long just once in the Premier League for Southampton, with the 28-year-old generally being brought on late in matches, when defences are tiring.
And O'Neill said: "Shane has come on against Poland [in March] and proved his worth [with a late equaliser], and I think at this minute the role suits him.
"When I get into the dressing room, I'll say 'well done' to him because he's won the game for us."
Long's pace and energy makes him a natural impact sub, as he is a nightmare for defenders to mark when fatigue is setting in.
His electric speed makes him a real weapon, and his skill set was on full display at the Aviva Stadium as he latched onto a loose ball and hammered it beyond Manuel Neuer in the German goal.
Long now has six goals in 11 appearances for club and country this season, and he will be hoping to prove that he is capable of making such an impact when starting in the future.