I have the same problem and yes, oblique work can make your waist thicker. I do obliques once a week only, & I avoid weighted oblique work since that builds the muscle.
Pilates is really good for toning the obliques without building them.
Another problem is heavy squats--they will also increase your waist size, but unfortunately this is one that can't be avoided for me since I'm trying to build some shape in the rear.