¡La Esquina de la Gramática! Rule 2

Accent mark in the letter “o”, no anymore-

Until recently, it was advised to place an accent mark in the conjunction “o” when it was written between two numbers in order to avoid confusion with the number “zero/ 0”. But that has changed. Most of us are aware of that change, but I know that some, who haven’t had time to be up to date with Spanish grammar, continue to place the accent mark.

Using the accent mark wasn’t necessary from the stressed and the graphic perspectives: the conjunction “o” is atonic or unstressed (it is pronounced without an accent), and the blank spaces before and after the “o” as well as its different shape and height than the number “zero” are enough differentiators to avoid confusion (1 o 2 vs. 102).

This updated rule applies when the conjunction is placed between words, numbers or symbols (¿Quiéres té o café?; Terminaré dentro de 3 o 4 días; Escriba los signos + o – en la casilla correspondiente).

Source: http://www.rae.es