They are all over PDC--almost in every corner! Oxxo is like the 7-11 store in the USA. It's a good store if you need a drink or snacks. They also sell a number of healthy items including juices, integral snacks, yogurts, etc.
The thing I have found is that Oxxo often has prices as good as major supermarkets, especially Walmart. I often buy milk and ice cream there. You will see some 7-11´s here in PDC but they are much more expensive than Oxxo. This particular store is near the beach.
A bit of Oxxo trivia. They were originally founded in Monterrey in 1978 by Grupo FEMSA as an outlet to sell their beer brands. They now have more than 11,000 stores in Mexico. read more