From my understanding oxygen is the by product that plants, for lack of a better word, exhale. I thought they basically ingested carbon-dioxide and then released oxygen as a byproduct. Am I wrong on this?

That's correct. The inverse square law puts a lot on energy from the Sun into Venus' atmosphere compared to Mars. Venus also has huge amounts of sulfur in the atmosphere as well.

Here is an interesting blurb from Wiki:

Unlike Earth, Venus lacks a magnetic field. Its ionosphere separates the atmosphere from outer space and the solar wind. This ionised layer excludes the solar magnetic field, giving Venus a distinct magnetic environment. This is considered Venus's induced magnetosphere. Lighter gases, including water vapour, are continuously blown away by the solar wind through the induced magnetotail.[3] It is speculated that the atmosphere of Venus up to around 4 billion years ago was more like that of the Earth with liquid water on the surface. A runaway greenhouse effect may have been caused by the evaporation of the surface water and subsequent rise of the levels of other greenhouse gases.[7][8]

Despite the harsh conditions on the surface, the atmospheric pressure and temperature at about 50 km to 65 km above the surface of the planet is nearly the same as that of the Earth, making its upper atmosphere the most Earth-like area in the Solar System, even more so than the surface of Mars. Due to the similarity in pressure and temperature and the fact that breathable air (21% oxygen, 78% nitrogen) is a lifting gas on Venus in the same way that helium is a lifting gas on Earth, the upper atmosphere has been proposed as a location for both exploration and colonization.[9]

On January 29, 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to "the ion tail seen streaming from a comet under similar conditions."

Nope. You are correct.

So they wouldn't really need for there to already be oxygen to start plants of the right type right? As long as there was water and other nutriments in the soil?

if your can find plants that grow in -50 temps...

Venus, Earth and Mars all had healthy atmospheres by our standards, most likely (you were initially correct as has been pointed out there is most likely plenty of water on mars at the ice caps and below the surface…and remember naughty mars wanders on its axis tilt (obliquity 25 degree currently) 0-60 degrees over millions of years, as opposed to earths constant(ish) 22-24 degrees.

So erm there is plenty of CO2 but no ozone and a very thin atmosphere and water – is a tricky blighter and something called the triple point of water comes into play – causing the water to sublimate away, above the surface, even when the temperature exceeds 0 Celsius.

So you could (un)safely plant around the equator. Below the surface is most likely ice or damp or wet ( I don’t know the depth at which it would be liquid water, not ice, nor permafrost for rooting etc-in some instances it would be barely below the surface)

And say plant a few billion trees for instant – which if my memory serves me correctly the amazon rain forest absorbs as much oxygen at night as it gives off during the day

Erm, so for oxygen I think we are going to need algae and most likely liquid water, or some hardy plant that produces more oxygen than it intakes and plant a few tens of billion.

And if you produce oxygen then you will produce O3 (ozone) naturally. (Ozone is produced naturally in the stratosphere when highly energetic solar radiation strikes molecules of oxygen, O2, and cause the two oxygen atoms to split apart in a process called photolysis. If a freed atom collides with another O2, it joins up, forming ozone O3).

plants aren't the main factor in producing oxygen, bacteria and protoplasm's are...they were the first life forms on earth, and still the most populous today

Protoplasm? Geez