You can not get over watched banned from only playing casual. Here's how it works, if you get reported enough in casual, one of your competitive matches will get thrown into the overwatch pool.
For example (This example implys that you need 6 reports to get thrown into the overwatch pool):
I play a legit game of MM and get reported once, I then go on to rage hack in casual / non-MM and get reported another 5+ times, my MM game were I played legit will get put into the overwatch pool.
Short version: If you don't hack in MM, you will not get overwatch banned.