Let's start from the and. As of rule of thumb which will help you to memorize cases whether you should pronounce
о (strict phonology purists, don't judge me hard, you know what I mean), strictly speaking there's no such rule of thumb.
One can say, wait, actually there is such rule, since
o is pronounces like
а only in unstressed positions, but this is quite delusive, since that actually means that you should memorize the stress patterns. And actually there are some pattern, but I would strongly advise you to memorize each word you need to learn separately. The thing is that such patterns are by no means rules and do not cover all the cases. There's a classic triple зо́лото/боло́то/молоко́.
Well, but if one had insisted I should present him such rule, I guess I'd say: don't bother with stressing prefixes (so, сопричастность, соучастие - all of these is pronounced with
a, mispronounced words like помОщь or памащь will be excused to a foreigner); always pronounce
о in monosyllabic words (there's just no other place you can put the stress).
As for errors in pronouncing. Well, saying
а instead of
о where it is actually is pronounced is a very rude mistake which can make your speech unintelligible. No one says
ан instead of
o instead of
a also is quite a big deal. But pronouncing
о without reduction well be 100% understandable ti any native speaker.
Yes, as @Quassnoi had already mentioned, you will sound definitely outdated and theatrical but as to me, this is a common mistake each novice language learner is doing: trying to pronounce everything as native speaker does. Well, you won't. It takes years to get decent native-like pronunciation. But if your vocabulary is reach, you grammar is irreproachable, many of phonetics mistakes will be forgiven. Moreover, the number of such mistakes will surprisingly decrease with time.