• L’installation terminée, la ville attendit son évêque à l’œuvre.

This sentence is not usual; "attendre à l’œuvre" is very rare in modern vocabulary. People are much more likely to say "attendre de voir qqn à l’œuvre". (ngram)

• L’installation terminée, la ville attendit de voir son évêque à l’œuvre.

Expressions such as "Passer à l’œuvre", "se mettre à l’œuvre" and "attendre à l’œuvre" do not imply any work of art, any material result of applying one’s art or technology, although this can still be the case; "œuvre" has a figurative meaning here, and this locution means "to wait for someone to get into action", whatever the action (frankly jocularly you can even say, for instance of two bank robbers that "Ils ne passent jamais à l’œuvre (rob a bank) sans avoir fait une prière que leur succès soit assuré.").

No, this linguistic ‘shift" in the expression from the usage of "voir à l’œuvre" to "œuvre" is not a common feature of the language but it can be found in some contexts: if there a work of art some sort involved, there is no reason for not using the word "œuvre" that translates "work".

• Ils s’étaient finalement mis à l’œuvre et le résultat n’était pas sans splendeur ; la partie de charpente déjà assemblée avait la marque d’une œuvre d’artisans chevronnés.

Complement

After the revelation in a comment that this locution is found in Victor Hugo I think that this can mean "attendre de voir qqn à l’œuvre"; it seems to me that nowadays this wouldn’t be put in the manner of Victor Hugo. What has been said in the answer, however remains valid.

The use of that locution seems very rare: https://www.google.com/search?q=%22attendre+%C3%A0+l%27%C5%93uvre%22&safe=active&filter=0&biw=1056&bih=553. On ne la trouve ni dans le TLFi, ni dans le dictionnaire de l’Académie ni dans le Littré.

The expression attendre à l’œuvre is perfectly correct and still used nowadays, in particular in the sentence on l’attend à l’œuvre.

It means to be in the expectation, waiting for someone to act.

Attendre l’œuvre is a possible expression but has a different meaning. Here we expect a masterpiece to be produced.