You've made the cake. I've used olive oil in place of vegetable oil but only if it is a really mild oil. I've had some that weren't and the cake batter tasted "off" (you could taste the olive). I've also melted butter and used that if I have found myself out of veg oil.
Some extra virgin olive oils have a great deal of flavor and are best used in foods where that enhances the flavor of the dish. If a person wants to use olive oil that has little, or very mild, flavor it's best to use light olive oil which has been well filtered.
That said, in a chocolate cake I doubt too many people could detect even a strong olive oil flavor.
I use olive oil in almost anything, altho I am definitely not a purist.
"I have always had an aversion to the concepts of in style and out of style." ~Rose Tarlow
Of course, you can use olive oil - wherever the recipe calls for vegetable oil. But even if the recipe says butter, oil could be substituted. In parts of the world, only olive oil is available - if the climate is too hot, or too rocky - or no grassis available for feeding, then no cows can be grazed or kept - and thus - no butter. Well - in those parts of the world, it's usually the olive tree which provides the liquid gold for cooking - and even baking.