If the recipe calls for oil, butter, or any other fat, then you can try to scale the other quantities to match it.
If not, there's still a chance you can salvage it. The oil doesn't actually chemically react with the mix at all, it just "coats" it; in fact, most cake recipes use some amount of fat in order to slow gluten formation (i.e. stop the cake from getting tough and chewy).
However, if the recipe called for 1 1/4 cups water and you used the same quantity of oil instead, then that is going to be way too much oil for a single serving.
What I would do is double or maybe even triple the cake mix, make sure it's very well mixed with the oil, then add however much water you needed in the first place.
If you realized your mistake early (i.e. if the original recipe called for a lot more than 1 1/4 cups of water) then you might be able to just proceed as normal and end up with a very moist cake, but more likely you'll still need to add some more cake mix (and the corresponding amount of water) to compensate. I'd still double it, because otherwise you'll be stuck trying to figure out how to use the leftover mix.
Don't just add flour; the amount of water that the recipe calls for is proportional to all of the other ingredients in the mix, and it's virtually impossible for you to know how much extra water you'll need to compensate for the added flour. Besides, you'd only end up with a bland, flavourless cake if you did that.
If none of these options appeal to you then I'm afraid it's destined for the trash, unless you want to prepare your own cake mix from scratch and combine the two recipes.
If you are only over by a small amount (¼ cup or less) just add half the volume of flour(±). If you added a moderate amount of extra water(½ to 1 cup) add half the volume of flour and ½ tsp baking powder and sugar equal to ⅓ the flour. Cakes fixed this way will be slightly dry but serviceable, particularly given that you started with a boxed cake anyway.
Beyond that point just make a double recipe.
Best Answer
If you have a stove: Put a pan on the stove and use low heat to "toast" your cake.
If you only have a rice cooker: The rice cooker is only for "wet-cooking" like cooking rice or steaming. I don't know whether toasting food in the rice cooker breaks the cooker. Therefore I don't know how to fix the cake then. :(