Call O2 and check what their policy is but bear in mind that O2 has taken an average of more than two people to court every day since 2002 for unpaid phone bills, some of' which were as low as €250. (ref Irish Times)
They probably have a clause tying you in for at least 12 months, so if you're within this timescale, they would be entitled to charge you for terminating the contract!
In a review of debt judgments published in Stubbs Gazette, The Irish Times has established that more than 2,390 people and businesses have been pursued through the courts by O2 since mid-2002. Its competitor, Vodafone, took legal action against just over 200 people and businesses in the same period.