Heat the cooking oil gently in a pan.
Add the garlic, onion and black pepper. Cook gently until softened a bit, about a minute.
Pour in the vinegar and soy sauce. Stir.
Add the oysters. Stir lightly. The oysters will expel juices and you will see more liquid than the equivalent of the vinegar and soy sauce. When the liquid boils, turn off the heat immediately, scoop out the oysters and transfer them to a plate.
Reheat the liquid and cook until reduced to about two to three tablespoonfuls. In other words, concentrate the flavors, especially the oyster juices.
When the liquid is reduced, turn off the heat and add the oysters to the pan. If there is liquid (there should be) on the plate where you had placed the half-cooked oysters earlier, add that to the pan too. Stir to coat the oysters with the sauce. Transfer to a plate or shallow bowl at once so that the oysters do not continue cooking in the residual heat of the pan.
Serve the oysters at once or at room temperature.