Generally, nouns ending in -o are masculine and those in -a are feminine. I think that rule is familiar to everyone, if only through proper names like Mario and Maria. It fits the vast majority of -o and -a words.
Now for the exceptions:
mano is the only noun ending in -o that is feminine, apart from some modern ones like 'la radio'.
There are quite a lot of nouns ending in -a that are, or can be, masculine. Many of these are professions, where the word is invariable but its gender can change: il dentista = the male dentist, la dentista = the female dentist, and the same for il/la giornalista etc.
Other words in -a that are masculine include il problema, il sistema, il tema (theme, essay). These examples come from Greek originals that are neuter.
Then there are some curious specimens that are masculine in the singular and feminine in the plural, such as il ginocchio, le ginocchia, l'orecchio, le orecchie.