An "O-Home" is a unique invention in the now-defunct City of Heroes MMO.
It is short for "Oranbegan Home", and it is created by repurposing old passageways to (reportedly) the ancient underground city of Oranbega that were used by the Circle of Thorns along with all sorts of criminal elements.
Earlier attempts to simply seal off the passageways failed on the simple fact that the criminal element would simply re-open them when nobody looked. Time and time again, efforts to deprive the criminals of these passageways would only be temporary solutions. At best, the caves would be re-opened. At worst, new efforts to re-open them could cause further instability to the city above.
Posting guards was obviously impractical. There were way too many caverns, and it would require people being there 24/7.
Then the idea came up to repurpose the passageways into living spaces that people would want to reside in.
Most of the Oranbega caves had ventilation, which explained how they were able to use torches underground and not die from suffocation. Gas-powered or even wood-burning fireplaces would provide heat, light, and the means to cook. They also had running water. The only thing missing was electricity and the more modern methods of communication.
These "O-Homes" would be sold as low-tech or no-tech single-room dwellings. They are in the most secure places, near the entranceway to the caverns, and right in front of the areas that were sealed with quick-cement.
O-Homes were first seen in "Task Force Twilight" #4.