When telling time in spanish, what is the phrase used to indicate that it is quarter past the
hour?
oy uno
oy veinte
oy media
oy cuarto

Oy cuarto indicates it is quarter past the hour.

ah, so ustedes and nosotros are my 'go-to' if i am using spanish mainly in the states? interesting, but still slightly confusing, haha."

it seems that you are a little confused about subject pronouns. yo=i, tú/usted=you, nosotros=we, ustedes=you(plural), ellos/ellas=they.

in the case of vosotros, it is an informal replacement for "ustedes" in spain. similar to using "you all" in english.

even though it is not generally used in conversation outside of spain, it is used many times in literature. i would suggest having at least some familiarity to that form, at least to recognize what it is.