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Oxo-bio plastic is NOT intended to degrade deep in a landfill, but if it is landfilled it will continue to degrade so long as oxygen is present, but becomes inert under anaerobic conditions and does not emit methane, see the Oxo-biodegradable Plastics Association website.
By contrast "compostable" plastic can emit methane (a greenhouse gas 23 times more powerful than CO2) in anaerobic conditions in landfill.
Oxo-biodegradable plastic will fragment in the upper layers and will save some space, but a landfill is not an environment for which degradable plastic of any kind is really relevant. Once a piece of plastic waste has been collected and buried in landfill it has already been disposed of responsibly.
Landfill operators do not want material degrading unless the landfill is designed to collect the gases, which most are not - they prefer stability.
Actually, plastic should not be wasted by burying in landfill. If it is not suitable for recycling (and most consumer plastics are not) it should be taken to a modern incinerator, where the energy within can be captured and used.