OR gates have two bits of input and a single bit of output.
The output of OR gate is logic '0' only if both inputs are logic '0'. Otherwise, the output is logic '1'.
The truth table defines the behavior of this gate.
The function implemented by OR gates has interesting properties:
The function is symmetric. Thus, x + y == y + x. This can be verified by using truth tables. We use "+" to represent OR.
The function is associative. Thus, (x + y) + z == x + (y + z). This can be verified by using truth tables.
Because of these properties, it's easy to define an n-input OR gate.
ORn(x1, x2,...,xn) = x1 + x2 + ... + xn
That is, an OR gate with n-inputs is the OR of all the bits. This is not ambiguous because the OR function is associative (all parenthesization of this expression are equivalent).