Hej Inger!
John O'Donnell is a very common name for an Irishman living in the US in the early 1900's. I have searched at the Ancestry database for you (the US national census records) and there are just too many options, if you can't provide some additional information - like a birth date etc. Provided that your relative called himself Herman, I believe this may be him and his wife (however, Annie is listed to have been born in Sweden?):
1910 US census Winchester, Middlesex, Mass.
Herman Magnusson, Head, age 30, married for 3 years, immigrated in 1898, alien (=not US citizen), Upholsterer.
wife Annie, 25, immigrated in 1901.
/Susanne Åkerfeldt