Wrongly convicted of murder and sentenced to death, he was freed after almost two decades of hard labour at Scotland’s HM Prison Peterhead through the efforts of multiple journalists, lawyers, and writers, including Sherlock Holmes author Sir Arthur Conan Doyle.

Around 1893, possibly to evade military service, he moved to London, where he purportedly worked as a bookmaker using various names, including Anderson, before settling on Slater for official purposes.

[2][3] In December 1908, Marion Gilchrist, a spinster aged 83 years, was beaten to death in a robbery at West Princes Street, Glasgow, after her maid, Helen Lambie, had popped out for ten minutes.

[2] At his trial presided over by Lord Guthrie, whose summing up was highly prejudicial, defence witnesses provided Slater with an alibi and confirmed that he had announced his trip to America long before the date of Mrs. Gilchrist's murder.

[10] However, Slater's lawyers organised a petition that was signed by 20,000 people,[11] and the Secretary of State for Scotland, Lord Pentland, subsequently issued a conditional pardon and commuted the sentence to life imprisonment.

[12] The following year, the Scottish lawyer and amateur criminologist William Roughead published his Trial of Oscar Slater, highlighting flaws in the prosecution.

The prosecution's evidence and witnesses identifying Slater as a suspect, including maid Helen Lambie, were also criticized as fleeting and otherwise unreliable, prejudiced, tainted, or coached.

The contents of the book led the Solicitor General for Scotland, Alexander Munro MacRobert, to conclude that it was no longer proven that Slater was guilty.

[3] In the 1930s, Slater married a local Scottish woman of German descent thirty years his junior and settled in the seaside town of Ayr where he repaired and sold antiques.