The Republic of Ireland are looking to make history and qualify for their first ever European Championship after making their debut at the World Cup finals last year.
Gleeson’s side were comfortable in both legs against minnows Georgia but will face a much stiffer test against Wales, who came from behind to beat Slovakia 3-2 on aggregate after extra-time in Cardiff.
“Wales are a great team; we have played them many times and know what they’re like,” added North Carolina Courage player O’Sullivan.
“We know their strengths but look on the night, anything can happen so we’re going to come in focused, ready to go and give everything for the shirt.”
The Republic of Ireland beat Georgia on Tuesday in front of a record crowd of 8,745 at Tallaght and O’Sullivan expects another big turnout for the visit of Wales to the Aviva Stadium in the final round of the play-offs.
“The fans come out all year for us, they’re massive,” said the 30-year-old.
“We’re hoping they’re going to come out again for that play-off game, they are always loud. They have supported us through the tough times and the good times.”