<compose> + _ + [0-9]
superscript: ^ + [0-9]
O⁰… O⁹
The long-winded way is to use your OS's character map tool: find the glyph you want there and copy+paste. Under Windows 10+ there is the emoji keyboard (hit [win]+;) which also gives access to much more including super-/sub- script characters, which is a little more convenient than character map. Presumably other OSs have similar available too.
Better is to have support for a compose key sequence. Usually build in to Linux & similar, you just might have to find the setting to turn it on and configure what your compose key is. Under Windows I use http://wincompose.info/ and there are a couple of similar tools out there. In any case it is useful for more than super- and sub-scripts: accented characters & similar (áàäæçffñ), some fractions (¼,½,¾), other symbols (°∞™®↑↓←→‽¡¿⸘♥⋘»‱), and configurable too so you can make what you use most easiest to access (and if you are really sad like me you can do something https://xkcd.com/2583/ to type hallelujah too!).
On mobile devices a fair few “special” characters are usually available (though it depends what keyboard you have installed) via long-press on the right keys of the virtual keyboard.