When you don't have a CO creature, if the game needs to select one, it can only be selected from your deck, not from your hand.
Current competitive CO decks include Pack Priest because this card allows you to actually select which card you want to be the CO. This card from the latest expansion was a huge improvement to CO decks. Then you include a high health creature such that it would have 10+ Health with the Pack Priest buff alone, which allows you to use immediately cards like Bearer of Change, Path of Light, etc.
CO decks are not very popular in Ranked. They work much better in Formats, where the absence of strong aggro cards slows down the aggro decks and allows for midrange decks like CO decks to have more space to breathe.
Here's a couple of good lists:
https://gudecks.com/decks/GU_1_3_BBqBBqKDbKDbLALMAPMAPNBHNBHNBONBTNBTNBUNBYNBYNBZNBZNBaNCeNCeOABOADPAfPAgPAmQADQADQAnCBlCEd?godPowers=100115&creator=%3C#FCBA03%3EAI%2099&userId=1860715&archetype=Control%20Light