In OO Math create an equation:

A^{a over b}

save it to file (e.g. test.odf), unzip this file, open contents.xml in a text 
editor and remove this part:

<math:annotation math:encoding="StarMath 5.0">A^{a over b}</math:annotation>

Now zip the edited contents.xml back into the created test.odf file and reopen 
in OO Math. The equation shows up as:

A^a over b

This problem is important if equations are created by some external tool (e.g. 
tex4ht TeX to OO converter) which won't include an annotation with equation in 
"StarMath 5.0" format.