How do type I muscle fibers differ from type II fibers in their respective oxidative capacities? What accounts for those differences?

Type I muscle fiber is also known as "slow twitch oxidative" fibers. Type I fibers are used in lower-intensity exercises such as very light resistance work aimed at muscular endurance and long-duration aerobic activities.

Type 2a muscle fibers are fast twitch, meaning they fire more quickly. They are also more powerful than type I fibers and are recruited for activities that require more intensity: sprinting, lifting heavy weights.

Type 1 muscle fibers are slow twitch and they use oxygen.

For type 2 there is type 2a which can use oxygen or glycogen, and there is type 2x which uses only glycogen. Type 2 is fast twitch muscle fibers.