Since I had a dream about me finding a method of transforming CO2 into oxigen I can't think at anything else beside that.

Do you really think it's posible? Maybe some day can we create a sintetic photosyntesis that will also do this more fast, more efficient and also in night time or why not in dark environment with some source of energy similar with the one that sun provides? Actualy photosyntesis relese CO2 back through respiration process. Can we somehow find a way to just avoid that with some process?

I want you to think some method even If its not viable just close to this as more opinions can bring more enlightment.

genetically engineer a plant that produces 500x more oxygen than a regular plant that can grow fast in a lot of environments.

You somehow know the method? I am intrigued in finding an answer.

from what i remember from my biology class it takes like 400 average production plants to produce enough oxygen to offset an adult humans oxygen usage. based on that if we could create a plant efficient enough to process oxygen at a much higher output we would only need 7 billion of these plants dedicated to oxygen production for the entire planet. that sounds like a lot but consider that the amazon rainforest alone has something like 300 billion trees alone.

I believe I saw an example of this in my chemistry lecture the other day but iirc the reactants were very expensive

They have been working on this idea for a while now, same thing happened to me when I thought about wireless electricity, turns out MIT have been working on it for years.

You should check out how a scuba rebreather works, or the scrubbers on a nuclear sub, or how the shuttle does it. Might not be exactly what your after but might help.