Learn Italian: O porti meno cose, o lo zaino pesa troppo. - Either you bring fewer things, or the backpack weighs too much.

While a single o just means "or", pairing them as o ... o translates to "either ... or". You put the first o before the first option and the second o before the alternative. Notice that in this sentence, it connects two entire actions (clauses) rather than just two nouns.

English uses "fewer" for countable items like "things," but "less" for things you cannot count. Does Italian do this?

No, Italian is much easier here! The word meno covers both "less" and "fewer". It is invariable, meaning it never changes its ending to match gender or number. So meno cose perfectly translates to "fewer things."

Why is the article lo used with zaino instead of il?

As a quick review, masculine nouns that start with z (as well as s plus a consonant, gn, and ps) require the definite article lo. If it were plural, it would become gli zaini.

Since zaino is masculine singular, is that why troppo ends in -o?