I was watching (only briefly) C-Span 3, Lectures in History - Guerrilla Warfare in the Civil War by Brown University Professor Megan Kate Nelson (not my favorite TV lecture) and she pronounced his name differently. I had always heard it with a long O. She gave it more of a "Maaaaasby" pronunciation.
"Moes-bee" is how Virginians (and Mosby was a Virginian) pronounce it and how I've always pronounced it.....
Don't know why folk pronounce it "Maahsbee"..... Yankees pronounce it differently -- there is no "h" is Mosby.....just like some say "Warshington." We say, "Waahshinton."
"The Past is a foreign country...they do things differently there" - L. P. Hartley