The pair are in line to debut in tonight's Capital One Cup clash with Morecambe.
O'Neill said: "(Johnson) is the type of player I love, someone who takes opponents on.
"He has been so exciting and just wants a platform to show it.
"Fletcher gives us a new dimension. I would love to promise that we will score a lot more goals with these two in the team but I do get the impression we will create more chances."