How to pronounce the article "o"?
At first I thought it was just pronounce "oh" but then in some instances it's pronounced "oo" like when paired with "sapatos". I keep missing it in the pronunciation tests because I can't tell when I should pronounce it as "oh" or as "oo". Is their a rule for it's pronunciation? Por favor e obrigada!
An UNSTRESSED "O" is pronounced closed to the English "OO" when fits in those requirements.  It is the last vowel of a word (this includes the definite articles "o" and "os");  It does not appear with a diacritic: "ó" or "ô".  It is not followed by a consonant except the letter "S" that marks the plural (otherwise, this specific "O" becomes stressed).
I remember some exceptions to that "rule":  The preposition "por" like in "por isso", "por favor", etc., is unstressed and pronounced with "OO" (in most of the accents). It is different from the verb "pôr", which is always stressed.  When someone tries to emphasize something (I don't remember if it's the case in those lessons);  Accents influenced by other languages like Spanish and other European languages: mostly some people from the South of Brazil, from the borders with some Spanish speakers countries or from other European colonies.
The rule applies to Brazilian Portuguese, but, as far as I remember, the same applies to the unstressed final "O" in European Portuguese. If anyone has something to add, please do it.
If any questions remain, please let me know.
Talking strictly about articles, either pronunciation is fine, much like the definite article "the" of English, which you can pronounce "the" or "thee". It varies by region and accent but, at least in Brazil, you'll mostly hear it as "oo".
- "Oos" sapatos.
- "Oo" carro.
Now, coming to think of it, you may prefer to pronounce "oh" when you want to emphasize the noun, as you would with a hard "the" in English. It's honestly quite arbitrary and you shouldn't worry too much about it.