Out of interest what is the exact reason you want to gain size in the obliques?
The most basic solution I can think of is to train without wearing a belt
My bodyshape is like an hourglass and women are supposed to have that not men, instead of the obliques going straight down they go inwards.
Its kind of like the first one, except obviously not as bad
Ps: this thread is not a joke and i am 100% serious