We do not have the technology to kill all oxygen producing species even if we tried. We some (many) species do die off, there will be room for other species to

We do not have the technology to kill all oxygen producing species even if we tried. We some (many) species do die off, there will be room for other species to use the resources the extinct species were using and increase in population. This change would have dramatic effects on the entire ecosystem, but deoxigination is not one of them.