"Politikus" and "politikusak" - when it is an adjective. "Political" and they "are political".
"Politikus" and "politikusok" - when it is a noun. "Politician" and "politicians".
And for the last "a", before the "t", I believe it will never be an "o", with any word. The suffix itself is either "-at" or "-et", when attached to a consonant. Or let's say the buffer vowel can only be "a" or "e" in this case. Somebody correct me if I am wrong.
Even for a word with only "o"'s:
doboz - box
dobozok - boxes
dobozokAt - (accusative) boxes
The same phenomenon can appear, for example, with the word "piros":
piros - pirosak - adjective
piros - pirosok - noun
When you pluralize a back-vowel noun, with some (many) exceptions "o" is the link vowel, just like here: politikus -> politikusok.
When you add the suffix "-t" to a noun in plural, only "a" (for back-vowel words) or "e" (for front-vowel words) can be the link vowel: politikusok -> politikusokat
By the two languages being different. Languages weren't meant for transcribing from one to the other. "any" is a particularly difficult word to deal with in all languages I know - in this sentence, "nem ismerek politikusokat" perfectly implies that the amount of politicians I know is zero.