My question is : In small spaces like cars or small rooms, how can aircons take in fresh air? In some cases, I feel tired for lack of O2?
You won't feel tired from poor air quality unless you have a very tightly-sealed box. And it isn't lack of O2 that causes it, but too much CO2. I once did a study on a school that was unventilated, in violation of code. The worst CO2 concentration I measured was something like 2500 ppm. Typical ambient is 400-600 and you don't start feeling the effects of high CO2 until 10,000-20,000.
Cars have an air intake that provides fresh air. The worst condition you usually have if there is insufficient fresh air in a car is high humidity. If your car allows manual control of the fresh air (usually a pushbutton with an icon for recirculating air or bringing in fresh air - an arrow running through the car or looping around in it), try this next time you are out on a cold morning (30-40F): Leave the fresh air shut off for a while - you'll notice the windows start to fog up and you'll notice the air feel "heavier" to breathe. Then open the outside air intake. You'll quickly notice the humidity drop.
In small rooms, it depends on how small and what kind of HVAC you have. If it is a through-the-wall air conditioner, most have outside air capability, but I wouldn't ever use it: unless you have a lot of people in the room, a house is not tight enough to trap much moisture or CO2, which is why codes do not require ventilation in a house (that and they typically have operable windows).