Is there any specific rule for why the word "Büro" uses the "das"-Artikel or is it just one of those "just memorize it" words?
Am I misguided considering that many words ending on -o are neuter in German?
das Chateau [okay, probably in analogy with das Schloss]
das Stroh [?! - probably only coincidentally]
das Holdrio [yes, confirmed by Duden; not to be confused with der Hallodri]
das Bordereau / das Bordero [however, Duden accepts both das Bordero and der Bordero]
der Bardolino (may also be because of der Wein)
der Risotto [Duden says it is der, although where I grew up in Germany it is definitely das - which Duden calls Austrian]
So, perhaps, words ending on -o unless they are Italian? No! Because of
das Diminuendo, das Crescendo
So, words on -o, unless they are Italian food?
Yes Büro derives also from the French. In the 19th century it was still written Bureau even in German, cf. Brockhaus. Note, that the word can mean the room as well as the group of people working there, see Duden, e. g. in
Das ganze Büro war in Aufruhr.
Especially in the latter meaning neuter is really convincing.
If in doubt you can fall back on.
Der --> männlich
Die --> weiblich
Das --> neutrum
As a Büro is not a living thing and therefore gender neutral you could call it das. But that is just one of the many rules that you could use to explain something that essentially follows culture/tradition.
Also see der Arbeitsraum vs. das Arbeitszimmer. Both refer to the same thing, just using a different compund word which in essence then determins the article being use.d