The Opera House is a legendary structure said to have existed in the City of the Gods on Kobol, alleged in the Sacred Scrolls to be the birthplace of Humanity, along with other edifices such as the Forum and Temple.
These are the first ruins on Kobol that viewers see after Crashdown's Raptor, damaged by a Cylon Raider attack as they headed to explore the planet, crash-lands on Kobol near the Opera House. Among the surviving passengers on that craft were Galen Tyrol, Tarn, Seelix, Cally, Socinus, and Gaius Baltar.
Shortly after his Raptor crash on Kobol, the virtual Number Six leads Baltar towards the ruins of the Opera House, where viewers and Baltar see the Opera House as it was, millenia ago. It is in that virtual Opera House that Six shows Baltar an image of what she claims is their future child (Kobol's Last Gleaming, Part II).
Much later a Number Three would have similar visions of the Opera House and beings of light therein, who she believed to be the final five (Rapture).
The opera house is also seen in a dream or vision by President Laura Roslin, induced by taking of chamalla. In the vision, Hera Agathon is scampering through the halls, pursued by both Roslin and her mother, Sharon Agathon (or another Number Eight in a flight suit). But before either can reach her, Hera is picked up by a Number Six wearing a white dress, ending the dream (Crossroads, Part I).
Just before the Fleet approaches the Ionian nebula, Roslin visits the opera house again, this time due to the diloxin treatment. She and Sharon Agathon are still chasing Hera through the lobby. However, when a Number Six picks the child up, she takes her through a set of doors. Roslin wakes up violently to find that Sharon shared the vision. Sharon and Roslin together question the captive Caprica-Six, confirming that she, too, had shared the dream. Later, while at the ionian nebula, Caprica-Six has yet another vision in the opera house, where she, Gaius Baltar, and Hera see the robed Final Five (Crossroads, Part II).
In the vision of the opera house shown Baltar by Six, there are 8 white drapes. When Number Three sees her vision of it, there are 6 drapes, with 5 glowing white-robed figures standing on the rightmost 5 (from her vantage point.)
Notes
- The Orpheum Theatre in Vancouver (where Battlestar Galactica is produced) provides the setting of the Opera House interior that Gaius Baltar sees in his dream (Source: The Official Companion).