I use the compose key (which I mapped to Scroll Lock) for subscript: <compose> + _ + [0-9] For superscript I use a dead key, ^: superscript: ^ + [0-9] O

I use the compose key (which I mapped to Scroll Lock) for subscript: <compose> + _ + [0-9] For superscript I use a dead key, ^: superscript: ^ + [0-9] O₀ … O₉ O⁰… O⁹