A versatile attacking midfielder who can play through the centre or out on either wing, O'Sullivan is another player who has come through the Rovers Academy to be pressing his claims for a place in the first-team squad.
One of the stars in the Under-18s run to the final of the FA Youth Cup in 2011-12, Dublin-born O'Sullivan made his Rovers debut at the end of 2012-13 as a second half substitute in the final game of the Championship season against Birmingham City.
An injury at the start of the 2013-14 campaign set O'Sullivan back, but he ended the season strongly whilst on loan at Conference side Southport where he scored three goals in 21 appearances for the Sandgrounders.
Former Southport boss John Coleman returned for O'Sullivan at the start of the 2014-15 campaign, taking him to League Two Accrington Stanley on a 93-day loan deal. The Irishman repaid his manager's faith in him by scoring twice on his debut, before going onto score a further two more goals in 13 appearances for Stanley.
At the end of the January 2015 transfer window the midfielder made the step up to League One with a loan stint at Barnsley. Making eight appearances for the Tykes during a successful spell before returning to Ewood to make two appearances at the end of the season.
A regular on the international stage for the Republic of Ireland, O'Sullivan has been capped up to under-21 level.