used to link alternatives

a cup of tea or coffee

are you coming or not?

I either take taxis or walk everywhere

it doesn't matter whether the theory is right or wrong

introducing a synonym or explanation of a preceding word or phrase

yoga is a series of postures, or asanas

otherwise (used to introduce the consequences of something not being done or not being the case)

hurry up, or you'll miss it all

introducing an afterthought, usually in the form of a question

John's indifference—or was it?—left her unsettled

either

to love is the one way to know or God or man

a logical operation which gives the value one if at least one operand has the value one, and otherwise gives a value of zero

gold or yellow, as a heraldic tincture

a bend or

operational research

Oregon (in official postal use)

other ranks (as opposed to commissioned officers)