OK, we're stretching the definition of opera a bit today. (But if you can perform Steven Sondheim musicials in the opera house, as many companies do nowadays, then anything is possible.)
Samuel Barber did compose operas though; two of them, in fact - the memorable and moving Vanessa, and the not-as-bad-as-everyone-says Antony and Cleopatra. He's probably best known, though, for the Adagio for Strings, op. 11, a magnificent piece that turns up on movies from The Elephant Man to Platoon, any time someone wants a tearjerker moment.
Less well known, perhaps, is Barber's own vocal transcription of the Agnus Dei using the theme of the Adagio. It really is a beauty, suitable (IMO) for liturgical use, and it makes one wish that Barber had composed an entire Mass around it.
Sorry I couldn't find a better video for it, but I chose to go with musical rather than visual quality. (Although if you like your pictures of Sam Barber holding a pencil, this one is for you.) This is the choir of Trinity College at Cambridge. ◙