oss.precision(5); That should do it, according to this page.
So what you would want to do is this:
oss.precision(5);
oss << ClassMembers.SomeNumber << endl ;
Mind you the "precision" implies the maximum number of digits to display (not the minimum, so it will not include leading/trailing zeros or spaces).
If you want the precise number of decimal places to be used, use std::fixed beforehand, like so:
oss.precision(5);
oss << std::fixed << ClassMembers.SomeNumber << endl;