In Texas co-op means that you become a member of the co-op, which means you become an owner of the company. It works like a mutual insurance company (ex. State Farm), you pay a water bill based on usage, but the co-op will return money to the co-op members if there is a surplus over the needs of the company. There are some small water co-ops, but I have found many listings confused and S.U.D. with a co-op. In this area, co-ops often serve only a very local area a few streets close together.
Red Oak is primarily served by Red Oak City Water or Rocket Water SUD. The latter is a special utility district. It is not owned by a government, nor by the users. In practice it works like water companies you will be familiar with in other areas.
I believe that you are referring to Josey Acres in Red Oak. Both of the above serve parts of Josey Acres. I've included the websites for Red Oak Water Dept. http://www.redoaktx.org/category/city-services-and-utilities
and for Rockett Water below.