Adding oxygen doesn't maintain carbonation, and this is even if the oxygen addition is pressurised. Only CO2 will help to maintain actual carbonation, as it's carbon dioxide which is released during fermentation. Adding additional CO2 as the keg empties both creates the pressure required to get the brew out of the keg, and ensures the brew doesn't go flat, and doesn't change the flavour of the brew in the way that other non-inert gases might.
That said, there are a number of small keg devices where you can use a built in hand pump to push the brew out of the keg. You are less likely however to notice oxidised flavours at first, and more likely to find the brew starts to taste flat. This is because oxygen is released more quickly from the beer than the CO2, and in doing so is likely to "pull" some of the CO2 out with it.
All that said, some ales can benefit from a small amount of oxidation, particularly when they've been formulated to be drunk without additional carbonation, however these are brews that generally don't keep well (agitation increases oxidation), and so they need to be drunk relatively soon after kegging. These should generally be dipped or pumped out, rather than pressurised, to reduce introducing additional O2 to the brew.
You don't necessarily need to purchases a huge CO2 rig to maintain carbonation and pressure, although this is the optimal configuration in most cases. There is also the option - preferably for small kegs of 5L or less - of using an adapter for readily available Soda Stream gas bottles (or similar). If you're using larger kegs, you'll need a larger rig.