TL;DR: The "O" is completely unnecessary in the beginning of questions.
Update: Even when grammatically unnecessary, in some places the "O" will make the question more "natural".
You can normally and correctly say, to mean "what do you see":
- "Que você vê?"
- "Que [tu] vês?" (similar to Spanish "¿Qué [tú] ves?") [more common in EU]
Sometimes you will hear an also unnecessary/expletive "que", solely to emphasise the question:
- "Que você está vendo?" ("What are you seeing?")
- "Que que você está vendo?" ("What are you seeing?")
- "O que que você está vendo?" ("What are you seeing?")
Grammatically, I do agree with you. But after a couple of years of living in Brazil, I learned that omitting "o" will sound weird in conversation, and is considered bad style in writing. Also the expletive "que" is accompanied by present tense third person singular conjugation of "ser". So the sentence would be "O que é que você está vendo?" Of course, you wouldn't hear that "é" in a fast pronunciation. You actually wouldn't hear a lot of things in fast pronunciations in the state of Minas Gerais. Fun fun fun Brazilian Portuguese....
I had totally forgotten about the "O que é que você está vendo?", which is another (correct) way of saying the same thing.
But saying that 'omitting "o" will sound weird in conversation, and is considered bad style in writing' can be misleading and oversimplifies the subject, even considering only the Brazilian Portuguese.
Some natural examples where de 'o' is or can be completely dropped:
- "Que (cursing word) é essa?" (WTF is this?) ["O que" would not be valid.]
- "Que tipo de carro ele dirige?" (What kind of car does he drive?) ["O que" wouldn't be valid, but "Qual" would be.]
- "Que é isso?" (What is this?) ["O Que" would be also valid]
- "Que caminho ela tomou?" (Which path did she took?) ["Qual" would be valid either, but "O que" wouldn't.]
But, to make the matter simpler, you are correct: "o que" is usually what Brazilians expect to hear and read at the beginning of questions (even when unnecessary; I should have made this clear in my first answer). And I hope I haven't made the matters worse. :P