He unsuccessfully stood for the House of Commons as the Liberal candidate for County Clare in 1879: his defeat is said to have been due to his opposition to Irish Home Rule.

As Attorney General he was considered to be a highly efficient civil servant; even Arthur Balfour, the Chief Secretary for Ireland, who thought poorly of most of the Irish Law Officers who served under him, praised O'Brien for his hard work.

He showed great skill in "packing" juries in politically sensitive cases with jurors who could be trusted to convict, thus earning the nickname "Peter the Packer",[4] which stuck to him all his life.

A. M. Sullivan wrote that as a pupil of the great Chief Baron Christopher Palles he must have learned the principles of common law but, though intelligent, he was generally too lazy to apply them.

His only legitimate son having predeceased him, he died without male heirs at Airfield, Stillorgan, County Dublin, on 7 September 1914, his barony and baronetcy thus becoming extinct.

[citation needed] Maurice Healy in his own memoir The Old Munster Circuit described O'Brien as a man of considerable legal ability and great natural kindness, who was deservedly very popular.