Alkoxides are much stronger nucleophiles than alcohols. Alkoxides are formed when alcohols are treated with strong bases.
Protonated alcohols have much better leaving groups (water) than non-protonated alcohols (hydroxide). Protonated alcohols are formed when alcohols are treated with strong acids.
So as a general first impression, if an alcohol is participated in a reaction involving a strong acid, the OH functionality is probably acting as a leaving group. Look either for a good nucleophile in the system and a substitution reaction, or an α-hydrogen and an elimination reaction. On the other hand, if the reaction involves a strong base, the OH functionality is probably being deprotonated and acting as a nucleophile - look for something with a good leaving group for it to attack.