Difference between the oxygen/$CO_2$ ratio during the period of the dinosaurs and the present
Is it true that during the time of the dinosaurs, both the oxygen
  requirements (by all living creatures on the planet) and the CO2
  released by volcanoes were higher than the same type of oxygen
  requirements today and the CO2 pollution created by us and the
  currently active volcanoes?


I'm not quite sure what "Oxygen requirements" means.   It seems to me that atmospheric changes is a byproduct of life on earth more than a "requirement".  There's other smaller factors, such as 
volcanic activity.

I think the best way to address this question is to start before the dinos, around 360 million years ago and the start of the Carboniferous Period
when oxygen increased to higher levels than it had ever been and CO2 levels fell significantly, perhaps as low as 300 PPM, though estimates vary.

These lower levels of CO2 played a key role in the Karoo Ice Age

Something rather drastic happened around 251-252 million years ago but CO2 levels had started to rise before then, and the simple termite may have played a key role in the end of the Karoo ice age and rise in atmospheric CO2 .

When the Dino's came about (230 million years ago), CO2 levels were too high to have glaciers.   Whether that was due to the massive mantle plume, or the termite or increased volcanic or some combination of all 3 is hard to say, but CO2 PPM has risen quite significantly by the time the Dinos came.



Source

I recommend reading this part as it's more relevant to your question:  https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth's_atmosphere#Measuring_ancient-Earth_carbon_dioxide_concentration


  I want to know if there's some simple argument against cars and
  factories being responsible for global warming (while methane
  production and deforestation are left behind) - that's what I believe
  in (for now).


There's really no good argument at all.   it's worth pointing out that while some people don't like the man made climate change theory and a handful of scientists continue to question it, it has stood up reasonably well to the tests, and no alternate theory has even come close.

When the Koch brothers funded a study to prove the "urban heat island" effect, their study suggested the CO2 effect was even bigger than the IPCC's studies said it was.

There are a number of factors that can effect climate.   The Earth's orbital changes, ocean current changes, sunspots, volcanism, deforestation, snow cover and albedo, cow farts, land location, mountain ranges and their effect on rain and atmospheric circulation, but none of those effects are playing a significant role in recent decades. CO2 is the only driver that makes any sense at all and it's quite clear that the cause of the rise in CO2 is the burning of fossil fuels.