BitOr[, , ... ] gives the bitwise OR of the integers .
Integer mathematical function (see Section A.3.10).
BitOr[, , ... ] yields the integer whose binary bit representation has ones at positions where the binary bit representations of any of the have ones.
For negative integers BitOr assumes a two's complement representation.
See The Mathematica Book: Section 3.2.4.
See also: BitAnd, BitXor, BitNot, Or, IntegerDigits, CellularAutomaton.