OAT is Organic Acid Technology. ELC is Extended Life Coolant, which has OAT, which makes the ELC extended-life.
Here's the deal. Most, if not all ELC is designed for extended run cycles prevalent in cross-country trucks with heavy duty commercial diesels. Such engines are run nearly night and day, and the OAT helps to prevent corrosion under these conditions. It is generally not suited for vehicles that sit a lot and aren't run much, and it tends to become acidic under such conditions after a couple years.
That being said, OAT ELC is fairly generic in the sense that OAT can be mixed with most other OAT at this point, just don't mix it with ethylene glycol or it will cause a chemical reaction between the two that will cause the coolant to solidify and clog up your cooling system.