Are words like "oh","barked", or "rang" considered onomatopoeia?
Onomatopoeia are words that sound like their meaning. For example, buzz, crash, or tick all sound like the noise they represent.
For the three words you ask about, I would assign these meanings:
Oh - basically a confirmation of something, or a reaction to something, "Oh" changes its meaning depending on context and the words around it. "Oh!" he shouted has a different context than "Oh," he muttered. It's a short, flat word, and unless specified it's a short, flat sound as well, so I would say that it is Onomatopoeia unless the context makes it something else.
Barked - "Bark" is considered Onomatopoeia because a dog's noise is often short and hard, though other noises (woof, chuff) are better examples. "Barked" usually means that a person is saying something in a loud and harsh manner, so it's less of a direct Onomatopoeia than a descriptive one: that is, it gives the phrase it describes the sound it is meant to evoke. "Drop and give me twenty!" he barked sounds different than "Drop and give me twenty?" he asked.
Rang - this is just the past-tense form of "Ring," and so is just as much of an Onomatopoeia.
To sum up, many Onomatopoeia words depend on context more than on themselves.