I dare not say for 100% but take it as a good guideline that the plurals of adjectives would rather have "A" then "O". Nouns, on the other hand, will more readily have "O". And you can see both of these in this sentence.
Some words used to be adjectives but now act as nouns. In that case, they will also take the "O". The word "munkás" is actually a good example for this. It means "worker". Just like in English, it now acts as a noun. So that is why it is "munkásOK", not "munkásak".
Let's take a real noun:
asztal - table
asztalOK - tables
But do not take this as a rule because:
tál - bowl (noun)
tálAK - bowls (noun)
There are a few words that can be used both as nouns and as adjectives. Then they get ther "A" or "O" depending on their actual role:
politikus - politician
politikusok - politicians - noun
politikusak - (plural) political(s) - adjective
okos - smart
okosok - "(the) smart ones" - noun
okosak - (plural) smart - noun