0O (zero O) or 0-O may refer to
- 0o, 0_o, and 0.o, an emoticon that may refer to shocked, disturbed, or confused
- 0-O, abbreviation for 0-operand Instruction set
- 0o, or zero object, a mathematics term for a simultaneously Initial and terminal object
- 0O, also ZO, an abbreviation for zero order; see List of zero terms
- Zero-order hold, model of the practical signal reconstruction done by a conventional digital-to-analog converter
- Zero-order reactions, a Rate equation for chemical reactions where rate is independent of reactant concentration
- Zero order kinetics, a Rate equation for chemical reactions where rate is independent of reactant concentration
- Zeroth-order approximation, or 0th order; see Orders of approximation
- Zeroth-order logic, first-order logic without quantifiers