IEC 60417-5008, the power-off symbol (circle) on a button or toggle, indicates that using the control will disconnect power to the device. (0 or ◯ means off.) ... IEC 60417-5010, the power on-off symbol (line within a circle), is used on buttons that switch a device between on and fully off states.

Is the O or I on or off?

The line symbol means “power on” and the circle symbol means “power off”. The presence of both (I/O) on a push button means the switch toggles the power.

Which way is on on a switch?

The direction which represents "on" also varies by country. In the USA and Canada and Mexico and the rest of North America, it is usual for the "on" position of a toggle switch to be "up", whereas in many other countries such as the UK, Ireland, Australia, and in New Zealand it is "down." ...