The residual mask/hose odor was "foreign" and "odd", but not particularly offensive. I eventually got used to it, and even "liked" it. I associated it with a "clean" smell.
The above paragraphs seem a bit off-topic, but were included to indicate I did actually use, disassemble, and eventually abandon the SoClean.
As far as where the ozone is actually pumped...
In my machine, the "supply" hose entered through an adapter on the mask supply hose close to the machine, then into a short flexible hose into the water chamber. The mask went into the box. Pump started, ozone entered the water chamber, and flowed through the hose and mask. I never noticed any odor when the machine was operating. Note: My olfactory senses are not particularly sensitive...
SoClean in this way establishes no connection to the machine proper, or flow through the machine proper. Yes, some ozone probably back-flowed into the machine, but would have to back-flow through the air pump inside the CPAP machine, which is unlikely. Anything that got past the pump would eventually have to leak out through the CPAP air intake filter - which SoClean would have had to carbon-filter that to manage the ozone leakage. They provide no such mechanism.
I have not seen any SoClean adapter that connects to the intake filter/port, or establishes flow THROUGH the machine. Then again, I have not seen everything - kind of like me missing the arrows on the check valve assembly.
FWIW...