Shaun Aloysius O'Mara is an agent with the British Secret Service.
He works, at least during the time we follow his activities, for the well known and respected spymaster, Peter Quayle, during the dark days of the Second World War and immediately after; dark being the operative word here as the collection of stories revolving around Quayle and those working for him are often referred to as the 'Dark series'. O'Mara is the key operative carrying out Quayle's instructions in two of them and considering how capable and exciting that man could be, it is surprising that he was not used more.
We learn a lot about O'Mara the man from his ad hoc partner in the first recorded adventure, Kerr, who said of him, "O'Mara would charm you, would eat and drink with you, converse with you, play cards with you, win money off you, take your girl off you, swear unfailing loyalty to you, and if necessary kill you." While we do not specifically see him doing some of those things, we see enough of his actions to believe all of that statement.
"He was tall and very big. He reminded one of a bull, except that, when you looked at his face, you were conquered immediately by a deep and peculiar charm which can only live in a certain type of Irishman". We are also told that "Mr. O'Mara believed that the world was a very good place. Quite obviously, at any moment of the night or day he was satisfied with it. He was prepared to be pleased with life in practically any circumstances, and it is necessary to add that he was quite often as pleased with death. He found the men and women in the world quite charming. He liked men in general and if, in particular, there was one here or there that he did not like, then, inevitably Mr. O'Mara found some way of dealing with the situation. He adored women. That is to say he adored the idea of women in general; was prepared to adore any woman in particular, providing she came up to certain standards; could be absolutely and entirely ruthless with a woman-if such process became necessary-in exactly the same manner as he could be quite ruthless with a man. His eyes were of an attractive blue. His face, which was round, was fresh complexioned. He had an attractive moustache of the Ouida type, and his fair hair curled back from an intelligent forehead in waves which made most women envious. Mr. O'Mara was an exceedingly able person. He was one of those people who can do almost anything. He played the piano, rode a horse, was a good shot, could sail a boat. He spoke a considerable number of languages, though very few people were aware of the fact. He had a nice taste in clothes which were always suited to his own peculiar characteristics. He had a quick, cool, mathematical-albeit whimsical-brain and was extremely apt with a hand gun."