It makes no sense to me; when I'm listening to Russians on YouTube they all do it either way, another thing. If I choose to do most of my o’s as a’s will there be others who don't still understand me?
This is a phenomenon called vowel reduction.
A good starting point would be the Wikipedia article on Russian phonology: https://en.wikipedia.org/wiki/Russian_phonology#Vowel_mergers
In a nutshell, most Russian dialects distinguish о and а and pronounce them more or less the way it says on the tin when they are stressed, and merge them (pronounce a neutral schwa instead of clear о or а) when they are unstressed.
There are some Russian dialects which (mostly) don't merge о and а even if they are unstressed, and those dialects are readily understood by practically any Russian speaker, though they do sound peculiar to those not accustomed to them.
You can't go pronouncing а instead if о or the other way around all the time. You have to separate them when they are stressed either way, and if you are unsure you can just always pronounce them as а and о, respectively, even if they are unstressed. You will be well understood.