So I designed an /o/.
It looks fine to me, but it's perfectly symmetrical (with respect to a 180° rotation), and since I know the upper half tend to look bigger than the lower one, I thought perhaps I should apply some optical corrections.
So I examined some /o/'s of well known fonts, comparing them to their 180° rotations, and found out that indeed none was actually symmetrical.
So what's the procedure? Create a perfectly symmetrical /o/, then just move up the right- and left-most nodes a little? That sounds naïve, doesn't it?
How do you progress once you have a perfectly stmmetrical /o/?
Well known /o/'s (in red) with their 180° rotations: