Cremation does not necessarily require oxygen. Heating the body in an electric furnace until all possible bacteria are dead should be sufficient.
The ashes can then be used in inside gardens.
Heat up the body in a furnace with no oxygen and you’ll get a desiccated mummy. There won’t be any ashes. You’ll drive off the water as vapor (65% - 85% of body weight), but that’s about it. All the original solids will be largely unchanged (except for maybe some calcining-type reactions).
You’d probably wind up with a shrunken head.