Oxygen generators are actually oxygen concentrators. They typically use a compressor. Compressors generally have high startup currents, usually above their rated wattage.
I would agree with at least a 1000 watt inverter. As to a sinewave converter, I doubt this is needed for running a compressor. As well all the internal electronics will convert the AC to DC and should function just fine.
Our Inverter is a Black and Decker that only cost us $59 at the Black and Decker outlet store. It is not a sine wave converter. We've used it to run everything in the trailer without issue. As well we have used it to run a small AC refrigerator
with a compressor.
It's apparant that you will not have an AC connection and will be running off batteries. I assume the concentrator does not have to run continuously, 4-5 amps at 120VAC is a lot of current for any battery