Burnley had an offer refused recently but we've been back to the Nottinghamshire club with a second bid that has been accepted by their manager Paul Cox.
Cox said today: "Luke will be leaving. Yesterday an offer was accepted from Burnley. A lot of people will see it as 'we should be keeping our best players' but unless you're Manchester United, Barcelona or Real Madrid you're a selling cup.
"The offer received matched out value of Luke and the chairman (John Radford) has said that the money we receive can be used to spend on players. It's a move that will suit al parties. We will have a decent fee and Burnley will have a good player."
O'Neill has been with Mansfield for one season after leaving Leicester City. He made 44 league appearances for the Stags last season as he established himself as their first choice right back.
He played just once for Leicester whilst also enjoying loans with both Tranmere Rovers and Kidderminster Harriers before making the move to Mansfield.