Phytoplankton is what makes oxygen from CO2. Trees actually do not add much. IMO we should bioengineer phytoplankton to allow it to grow faster, so our oceans will just turn more CO2 into O2 and solve that issue.
... and in the process destabilize every other ecosystem in the ocean? I think that's a dangerous direction to go into without further insights into what kind unintended consequences there might be.
Well, the carboniferous era would disagree with you :)
I still have no sense of how feasible or efficacious that might be.