Why would a disposable plastic container, such as the one depicted above, need an O-U?
A number of kashrut certification agencies (led by Star-K and the OU) discovered that plastic packaging material suppliers were using animal-based additives (e.g., tallow, animal fat) in the plastic resins (e.g., as lubricants) and that these additives could migrate into the food. Interestingly the Food & Drug Administration requires these chemicals to be food-grade because of this interaction with food.
This can potentially represent a kashrut issue when heating up food in these packaging material so the Star-K and OU worked with the chemical suppliers to these packaging companies to switch from animal to vegetable additives and started putting kosher logos on these new packagings.
For further details see here from the Star-K.