I did Less Oxygen once while spending a winter in a tipi.

Snow on the side of the lodge slid down to cover the bottom while I was flaring the fire. The flames got small and odd looking and I was going to light a candle so I could see better.
The candle wouldn't light. The fire in the pit sort of burned, but the flames looked sort of transparent (can't really remember that part, just that they looked different) The candle wouldn't light, then matches wouldn't burn...
My thought processes and movements got slow but I had a thought that maybe snow had slid down to cover the bottom of the lodge (air circulation)
The lodge didn't fill up with smoke from the fire.

I didn't feel like I couldn't breathe. It didn't hurt at all, just made me slow.
When I opened the door and poked my head outside, I was perfectly ok after a couple breaths.