Ok, I hope you have noticed that you can add formulas to the presentations too in openoffice?
It looks quite neat.
You add it just as you do for the Writer documents, Alt + I, o, f and start typing. Obviously you need to adjust the size for slide presentations.
Don’t forget to look at those style definitions too. A very handy way to update and adjust figures globally. In addition to this you can sort your formats/styles hierarchically, such that all objects in the hierarchy tree inherits the higher level format/style definitions. Quite useful, if you want to switch between slide and document formats for your figures.