'''18O''' or ''variant'', may refer to:
* Oxygen 18 (<sup>18</sup>O), an isotope of oxygen with atomic mass number 18 (8 protons + 10 neutrons) * 2019 Catalan general strike (''18-O'') * 18º, a way to write the ordinal number 18th (eighteenth)
==See also== {{Canned search|18o|18-o}} * δ18O * {{intitle|18o}} * {{intitle|18-o}} * O-18 (disambiguation) and O18 * 18 (disambiguation) * O (disambiguation) * 180 (disambiguation) (one-eight-zero; 180) * I80 (disambiguation) (i-eight-zero; I80) * l80 (disambiguation) (L-eight-zero; l80)
{{Letter–number combination disambiguation}}