"Wa" is used to refer to the main topic but "o" is used to refer to actions, for example: 車は白いです。Kuruma ha(wa) shiroi desu.　(The car is white) あなたは学生です。Anata ha(wa) gakusei desu. (You are a student) 車を買います。Kuruma wo(o) kaimasu. (I will buy a car) 電車をおります。Densha wo(o) orimasu. (I will get off the train)

"Wo" is never used to indicate the topic, it is the most common particle used after the object of transitive verbs. (when someone is doing an action). "Mado wo akeru": I open the window. There are verbs like the noru, noboru and au, and others, that use special particles. (ni noru, ni noboru, ni au), For intransitive verbs you use the particle ga. "Mado ga aku": The window opens. Or more commonly, "mado ga aiteiru": the window is open. For these actions that happen by themselves (without someone "doing" them) you use "ga". As for wa, you can leave it out most of the time, since the subjects only need to be introduced once, or even not at all if they're clear from context. But yeah, it's the main topic. You'll have more trouble figuring out when to use wa and when to use ga than wa and wo.

the two words are used to refer to the main topic of the sentence are "wa" and "ga". o usually indicates the subject of a verb.