input/output, I/O, input/output control

Avoid input/output in content for a general audience. If you must use it, spell out on the first mention, and then abbreviate as I/O.

In content for a technical audience, you don't need to spell out on the first mention.

Use input/output control only in content for a technical audience. Always spell out on the first mention. On subsequent mentions, it's OK to abbreviate as I/O control or IOCTL.