OAT - organic acid/additive technology. These are coolants that say they are non-toxic, such as DEX-COOL. They do not play nice with ethylene glycol based coolants (IATs). When mixed, they form a sludge/gel that can block off water flow through the jacket. Not immediately, but over a relatively short amount of time.
For example - when they put regular coolant in my patrol car previously filled with dex-cool, it took 3 months of hard driving, but eventually it happened that I began overheating. They wouldn't listen to me when I told them what the problem was. They replaced fans, hoses, thermostats, and by the time they listened, it was bad enough that the entire radiator had to be replaced and the entire cooling system flushed many times to clear it. It's a miracle it didn't warp a head.