Bitwise OR on the two arguments. The arguments are converted to a number. A List, Dict or Float argument causes an error. Also see and() and xor(). Example:

:let bits = or(bits, 0x80)

Can also be used as a method:

:let bits = bits->or(0x80)

Rationale: The reason this is a function and not using the "|" character like many languages, is that Vi has always used "|" to separate commands. In many places it would not be clear if "|" is an operator or a command separator.