This is the 8th most frequent Portuguese word.
o
"The" (masculine singular definite article).
Here, 'o' is a definite article used before the noun 'livro' to indicate that it is a specific book.
O livro está sobre a mesa.
The book is on the table.
In this case, 'o' serves as a pronoun complementing the verb 'ouvir', representing 'what' or 'that which'.
Eu ouvi o que você falou.
I heard what you said.
Here, 'o' acts as a definite article preceding 'amigo', specifying which friend is being referred to.