Charging MacBook Air with a fast charger from usb-c

I have a MacBook Air 2020 and forgot my charger at work. Evidently I need to charge it in a few hours. I have One Plus phone with a fast charger which is USB-C compatible (5V, 4A). Could I use it to charge the MacBook Air? Does this damage the mba ?

It shouldn't damage anything, but it might not be powerful enough to keep up.

The Macbook Air charger is 30w, which is about 6A.

Amps are "pulled" by the computer rather than "pushed" by the power supply [which is the opposite to voltage; this is why voltage has to match but amperage doesn't] however, your charger will be incapable of supplying all the Mac is asking.
This will just make charging slower. If you're lucky, it might just keep up, otherwise the battery might still lose charge slowly while the Mac is in use, even with the charger connected.

I didn't think it would... but the ubiquitous Apple 12W USB-A iPad/iPhone charger

works fine for me when my M1 MacBook Air is shut down. The battery life is good enough to handle moderate use (take measures to minimize drain!) and then I just plug it in when I'm not using the laptop, shut it down and it charges slowly. (USB A to C cable needed) Definitely needs extra time but is usually charged in a couple/few hours.

I guess the only question I have about this would be - is there anything wrong with leaving that setup (12W USB-A charger, USB A to C cable, M1 MacBook Air) plugged in while I'm working on the computer? I have heard that it's best to not drain and charge at the same time. So I guess more generally I need to research the entire question of charging while using.

Apple laptops with USB-C, and the power supplies that come with them, will use USB-PD for charging. I've discovered that in a situation like yours my MacBook Pro will charge slowly from a 12 watt iPhone power supply that uses USB-BC and Apple's BrickID for charging.

Based on what I know from USB-PD I believe your One Plus charger does not support USB-PD. This is because 5 volts at 4 amps is not a valid USB-PD power profile. A bit of research tells me that this charger is likely using a charger protocol called VOOC or Dash Charge. These chargers will fall back to USB-BC charging for backward compatibility with older One Plus phones. The supported USB-BC charging rate from this charger is likely 5 volts at 1.5 amps. The 7.5 watts you will get from your phone charger is 1/4 what you would get from the Apple charger that came with the laptop.

I'm quite certain that your MacBook Air will take power from this charger to at least extend the battery life while in use, perhaps even have some power to spare for charging. I'm quite certain it will charge the laptop while it is powered down or sleeping. I'm most certain that this will not damage either the charger or the laptop, both have enough "smarts" in them to not do anything that will damage them. The transfer of power would stop before damage is done.

I'm quite certain my answer is too late to help with your dilemma of avoiding a dead battery, but will hopefully help you or someone else in the future. One last thing I'll say I'm at least minimally certain about is that it would be wise of you to invest in a spare USB power brick better suited for powering your laptop. That's assuming you didn't do what I did that forced me to charge my 60 watt laptop from a 12 watt phone charger, temporarily misplace two laptop chargers.

Years ago I found myself in a spot with a much needed laptop running low on battery and no charger for it in the vicinity. I keep a spare laptop charger on hand since then. Today this isn't near the problem as then since USB charging is the norm, chargers are less expensive now and very interchangeable. The problem we will find today is that not all USB chargers are equal. The chances of me not having any means of charging my newest laptop is near zero now as I have multitude of USB chargers on hand. The number of them rated for more than 12 watts though is just two.

I don't know if a 20 or 30 watt charger exists that supports both VOOC and USB-PD. If so, and it doesn't cost too much, then getting one might be a wise investment. A quality 20 watt USB-PD charger can be had for $20, and should provide enough power to avoid running the battery dead on a MacBook Air unless it's run really hard, and will also be a spare 7.5 watt charger for your phone. A $30, $40, or $50 charger removes the slow charging concern for the laptop, but not for the phone.

I do wish USB chargers would put on the charger what charging protocol they use, and do so using the same names as everyone else. VOOC is the same as Dash charge. Quick Charge is the same as PowerIQ. What Apple calls "fast charge" is just USB-PD by another name. It also doesn't help when they use such similar names for incompatible charging systems. Well, they are incompatible for providing more than 5, 7.5, 12, or maybe 15 watts of power. They will all at least fall back to providing something for power. There will be no harm done for people that mix up these USB chargers, but mixing the up will get disappointing charging rates.