As a general rule, most masculine nouns end in -o. But there are many other noun endings. There are some general rules you can memorize to help you recognize masculne nouns even if they don't end in -o.

Many masculine nouns end in -aje. These nouns are almost never feminine.

Many nouns ending in -ma, -pa, or -ta are also masculine.

LONERS nouns: nouns ending in -l, -o, -n, -e, -r, or -s are usually masculine.

Most of the time, a noun that ends in an accented vowel is masculine as well.

There are always exceptions, but remember these generalizations.