Astrochemical Thought Experiment

Precisely what would happen if, suddenly, 87.5% of a given gaseous planet's mass was somehow transformed into pure oxygen, and immediately after, five small asteroids of pure caesium, each possessing a surface area exactly equal to 5% of the gaseous planet's topographic surface area, collided with the newly oxygenated gaseous planet at the same time at equidistant coordinates? Precisely what would be the effects on the astrochemistry and meteorology of the planet, and how would it effect other nearby astral bodies? It's admittedly a vague question with many variables, so I'll provide some specification; according to what we know about the Solar System, what would be the aforementioned results for each of the gas planets (individually) if they were to undergo this process?