Thus it was that O'Brien, eleven in 1919, had his basic football education in Scotland, continuing it as a teenager in New Jersey's Scotstoun with local, junior clubs. Indeed with Ryerson he was to win the New Jersey State Cup before at seventeen being recruited by and turning professional with the New York Giants as a left-winger cum inside-left.

In fact Shamus was, living still in Kearny, to remain in New York football for eight seasons, winning two league championships, the second in 1932. But it was just as American professional soccer imploded economically and in 1933 he joined, at its foundation, the new hometown club, Kearny-Irish, winning another league title and playing on to 1938, aged thirty. Yet in that period of on-field success he was never awarded a cap for the simply reason that, even though selected for the 1930 World Cup, he was not an American citizen. He did not complete naturalisation until 1937, by which time he had been married since 1928, his wife Catherine Lane from Jersey City, had two children and had, off-field, been a clerk in a local flax mill before becoming an insurance agent and then working for the Prudential until retirement. And on that retirement he would move to Bangor in Maine, dying there in 1981 on the day before his seventy-fourth birthday, the funeral service taking place back in Kearny.