The spelling double O (OO) can have more than one sound in English. Usually it is either the long OO /u/, as in the word food, which is the same thing as the long U sound, or else it's the short OO /ʊ/ sound, as in the word foot. If you have to guess which one it is, guess the long OO /u/, because that pronunciation is much more common. However, there are a number of common words which have the short OO /ʊ/ sound, so it's good to learn them.
Here's an alphabetical list of the root words that have OO making the short OO /ʊ/ sound:
Now remember, these are the root words, so there will be lots more words made from these roots, such as childhood, cookie, football, understood, hoodie, outlook, goodbye, etc.
And the /ʊ/ sound has other spellings as well, including O, OUL, and U. Here's a list of words with these spellings of the sound:
Hope that helps!
Did I miss any words? Let me know in a comment if you can think of any more!