Like soy and other non-dairy milks, oat milk can separate and sometimes look "bad" or spoiled when added to a hot ingredient.
Assuming the "Sell by" or "Use by" dates have not passed, the key to stopping this is to temper the milk as you add the hot substance.
For example, if you're using a Keurig, add the milk and any sweetener to your cup first, then place the cup in the proper position and hit "BREW." Ensure that you account for the amount of liquid you pre-add so the cup doesn't overflow when you add the coffee.
If you pour hot water, tea, or coffee from a carafe directly into your cup, add the oat milk or other non-dairy milk and any sweetener to the cup first. Then, while stirring, slowly pour a few tablespoons of milk into the cup to reach the desired temperature. Finally, pour in the remaining hot liquid.
Tip: Always shake the oat milk container before use to eliminate any natural product separation within the carton.