Hi John

Well, I did look for an army record for Cyril, but the only one I found was for a Cyril Charles of Edmonton, who was the son of a Charles Smith, and he enrolled in Nov 1914, so not your one.
However, if the Edward/Mary Ann marriage is correct, then Osgood the witness appears to be Edward's brother:
1901 Census
RG13 165 62 6
87 Hanley Road, Islington
William SMITH Head 79 retired surgeon; b London Chelsea
Elizabeth Wife 76; b Middlesex Edmonton
Osgood Son 32; temporary civ draughtsman, (war office) ; b London Clerkenwell
KR
Maryam