"We do not know our grandmother."
Translation:Noi nu o cunoaștem pe bunica noastră.
It's the pronoun in accusative case for the feminine 3rd person singular (as in "pe ea" / "o", which roughly translates to "her"). For example: "I know her" would translate to "Eu o cunosc", or the more stressed/reinforced "Eu o cunosc pe ea". See also the sentence and comments here.
Not to be mistaken for the indefinite article "o" of feminine nouns (as in "o bunică", which translates to "a grandmother"). Confused yet? :-)
This stressed/reinforced way of communicating is very confusing to understand. People actually talk like this on a daily basis?