Back to the OP.

My wife has Win 10.The other day she was trying to enter 'O₂' (the UK phone operator) into Excel. After trying various keyboard combinations (she found O²) she gave it up and entered 'O2'.

I just mentioned this window + dot thing and indeed there is a subscript 2, so she entered O₂ into the field at the top of the Excel screen. Fine - except that it doesn't appear in the cell itself which has O followed by a box - which probably means unprintable. She also tried the superscript 2: that resulted in O" (which happens to be shift 2 on a UK keyboard).

She was unimpressed. The line stays as O2.