The MathML package (load mathml;) and the OpenMath package (load mathmlom;) are incompatible in the following way.
1. If you load the mathml; package, and then execute this code, it works as expected, outputting a polynomial.
<ci> x </ci>
<cn type="real"> 2.5 </cn>
<cn type="integer"> 10 </cn>
2. If you load the mathmlom package after loading the mathml package, however, the parseml() function of the mathml package no longer functions. Executing the same code as above instead yields the prompt
Declare power operator ? (Y or N)
(I have a workaround for this in my own code because I'm calling REDUCE as a subprocess in another piece of software. So I just run more than one REDUCE, and use the two packages in separate instances. But I suspect for 98% of users, that's not an option.)