Assuming you mean something like A2+B3=C5...
Type your text into Open Office - then highlight the individual characters you want to change (in this case it's the numbers)
Select Format then click on Character, and select Superscript - Click OK to finish. OpenOffice will raise the text and reduce the font size so it looks like the above example.
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I select the character(s) to be an exponent /subscript
On that tab I can adjust the size and how high / low relative to the normal character the superscript /subscript is to be (when making presentations I often exaggerate the raised height - to maybe 40% or 50%)
Highlight the text you want to change, click Format then Character, tick the 'subscript' option in the dialogue box that opens - then click OK. This will change the text you highlighted into subscript.