The Opera House, normally:
Bonus: the Harbour Bridge in the background!
You can see more pictures of it if you click on that link just above, and read about the history, the construction, etc. etc. if you want. Fun fact: the architect - Danish!
The Opera House, during Vivid Sydney 2014:
Photo credit: me.
The building is beautiful and, of course, iconic. Never actually made it to a performance within the walls, but really, that's okay.
Now that I think of it: O can be for one other thing:
This is an opal in the rough - still embedded in the rock it formed in.
And this is a fire opal ready to be set into a piece of jewelry. These guys are super expensive, when they are solid - (this one isn't, it's on Etsy!) and stunning.
Read all about them here: