¡La Esquina de la Gramática! Rule 6

Change of disjunctive “o” to “u”

In Spanish, when you have the disjunctive conjunction “o” before a word that starts with “o” or “ho”, the conjunction “o” should be replaced for “u”. For example: tigres u osos, minutos u horas, recibir u obtener.

Also, when the numbers 8, 80, 81, 800, etc. follow the disjunctive conjunction “o”, it should be replaced for “u” because those numbers start with the sound “o”. Example: 700 u 800.

The same rule applies if there is a conjunction before one or more sentences: Los hornos antiguos eran o circulares u ovalados; La disyuntiva era clara: u obteníamos beneficios pronto o habría que cerrar la empresa.

Source: http://www.rae.es