Career
He was a Senator from 1989 to 2011, elected by the Agricultural Panel. O'Brien is a native of Latton, near Ballybay, in County Monaghan. On 25 April 2010, the Irish Mail on Sunday ran a lead story saying that O'Brien had borrowed €10 million from Irish Nationwide Building Society, under the headline "FF Senator got €10m loans from Fingleton".
The article claimed that the loans were fast-tracked by Michael Fingleton "without demonstrable security or documentation". On 14 June 2010, the Irish Independent reported that in 2006, O'Brien spent millions of euro on agricultural land outside the village of Inniskeen, County Monaghan in the hope that it would be rezoned for housing. The land is now worth a fraction of the purchase price.
O'Brien did not contest the 2011 Seanad election. In November 2013, he was sentenced to two years' imprisonment and a further year suspended, after pleading guilty with two other men to a charge of demanding €100,000 with menaces from Michael Heelan at Carrickmacross on 27 April 2012.