The former Manchester City winger has been well below his best since his summer move from the Premier League champions but O'Neill feels scoring the opener in the eventual 2-1 defeat to the Toffees can help Johnson shake off the cobwebs.
"Scoring the goal was very, very important," said O'Neill. "I don't care who you are, the very, very best players can lose confidence, but eventually, if you feel as if you have the talent, you should pull through.
"How you do that is either getting down to doing things more regularly in training - there's always a possibility of being able to resolve it then - but the confidence really comes from performing on the field.
"I have said this before and it's almost as if it's a recurring theme, but players who are confident and who can make things happen when they are confident, it's a major boost to them when they feel as if they are capable of doing it."