O and U mean “or”, but their use follows some rules.

1. When O comes before words that start with an O or HO it must be replaced by U.

plata u oro (silver or gold)
minutos u horas (minutes or hours)

2. When O comes between numbers, we must replace it by Ó (with an accent) so it is not confused with zero.

8 ó 9 (8 or 9)
9 ó 8 (9 or 8 )
12 ó 15 (12 or 15)
8 ó 80 (8 or 80)
20 ó 30 (20 or 30)

When numbers are written out here’s what happens:

ocho o nueve
nueve u ocho
doce o quince
ocho u ochenta
veinte o treinta

Here’s what happens with Y and E (meaning “and”):

When Y comes before a word which starts with I or HI (followed by a consonant), it must be replaced by E.

naranjas e higos (oranges and figs)
fabricantes e importadores (manufacturers and importers)

However, when we have the diphthong HIE, we must use Y.

Añade leche y hierve. (Add milk and boil it.)
flores y hierbas (flowers and herbs)

Nos vemos prontito.