Hi, mrsmosey. Welcome to Discuss Cooking
Is this an oxtail stew? What other ingredients are in the slow cooker?
I don't see any reason why it wouldn't be fully cooked after 8 to 10 hours in the slow cooker. It should be on low the whole time.
To store it overnight, remove the crock with the food, let it cool and either cover it and put it in the fridge or transfer the food to another container and refrigerate. Reheat in a saucepan over low heat.