Does an onion completely remove "fresh" paint VOCs and other chemicals emitted?
I've never heard of onions, lemons or vinegar removing a paint smell, but I would guess that they'd just cover up the unpleasant odor with something nicer.

As to your later comment:


  I wouldn't want to remove the smell but leave the VOC or other bad elements there.


VOC stands for "volatile organic compound." (See the Wikipedia page on VOCs) Volatile compounds are ones that evaporate easily. Technically, the smell of that onion (or lemon, or vinegar) is a VOC, just not a particularly harmful one. (Not knowing the particular chemicals that make up a onion's scent, I can't say for sure that it's a volatile organic compound, but it's a pretty good bet.)

That fresh paint smell comes from VOCs (and potentially from other volatile compounds), so by the time the smell is gone, any VOCs you couldn't smell are likely gone too.