formal means with more respect. For e.g.. with bosses, parents, seniors etc Informal means a friendly conversation for e.g. with friends, juniors, in insults etc
Now "usted" is used as a third person (like él, ella, Juan, Ana , jefa, soldado ) and hence the third person verbs like oyó,comió,habló etc
whereas "tú" is used as a second person and hence the second person verbs like oisté, comiste, hablaste
It's a peculiarity of Spanish that they tend to use object pronouns even when the object is in the sentence. Here the object is "eso", and so there's no need to have an object pronoun, nor would it be used in formal registers of Spanish. In anything but the most formal registers though, it is common to use both the object pronoun as well as the object itself, so you would very often hear "dónde lo oyó usted eso", most often with stress on 'eso', much like you might say in English "where did you hear THAT?".
many translations given for this for oir when the same sentence was asked to be translated. from Spanish to English SHEESE !
I hate to complain. Yet so often Duolingo does not give any translation after completing a segment. The correct translation would be very helpful. Perhaps it is because i am supposed to be over 50% fluent now? When i wake around 4-5am my mind takes time to warm up. The translations used to be very helpful... the lack... frustrating.