"Oxigenated" and breathable aren't the same thing.
Venus' atmosphere is mostly carbon dioxide. You can fixate the carbon to graphite with relative ease using the Bosch reaction and electrolysis of the resulting water, wherein the hydrogen gets reused as long as needed venting the excess oxygen.
This has only the effect of reducing the habitats' buoyancy and starting lots of interesting oxidizing reactions down on the surface (which by the way will be 50 km lower, not 500).
The problem is that to be breathable the CO2 concentration must be less than 5,000 parts per million (down from the current 975,000), while oxygen concentration must not exceed 60%. We don't have anything available to make up the remaining 40%.
Magically converting 60% of the atmosphere to oxygen would have people die of CO2 poisoning, and converting 100% of the atmosphere would have them die of oxygen toxicity.
Freeing enough nitrogen from the surface of Venus that its concentration at 50 km is breathable also looks like a losing proposition - it's mostly aluminum silicates. Also, I'm not sure we could work a viable solution for O2 and N2 partial pressures at that height.
So, the simplest alternative would be to either build really large habitats, or equip them with very good projectors and external displays.
As @Rekesoft noted, Venus does have nitrogen in its atmosphere, it's just not concentrated enough. To use this, we need to get rid of about 15/16ths of the overall CO2 atmosphere (and then some more, because of the predictable outgassing), leaving in place all the nitrogen. This should give us a 80% N2, 20% CO2 atmosphere, about four times as dense as Earth's; at a sufficient height, that ceases to be a problem (but now buoyancy is). Proceed with the conversion of CO2 to O2 and graphite, and you're left with a breathable atmosphere, as long as you don't go too near the surface.