O'Toole made his Watford debut against Southend United in the League Cup as a 71st minute substitute on 28 August 2007. He made his league debut four days later as an 88th minute substitute against Ipswich Town. In September 2007 he signed his first professional contract, a one-year deal. He scored his first goal for Watford at home against Bristol City on 1 December 2007. Coming on as substitute in the 83rd minute, he headed a goal three minutes later to make the score 1–1, although Watford went on to lose 1–2. In the next game he started in place of captain Gavin Mahon and scored again, another header, in a 2–3 win away at Colchester United. His third goal for the club came on 26 January 2008 in a 4–1 home defeat against Wolverhampton Wanderers in the FA Cup 4th round. His performance the next weekend, once again at home to Wolves, won him a place in the Football League team of the week. O'Toole was sent off for the first time against Leicester City on 12 February 2008, although his red card was subsequently rescinded following an appeal.