OneNote can insert an equation like in Word.
You just simple click "Insert" tab -> "Equation" command to insert an equation.
or simple press Alt + = keyboard shortcut to insert an equation.
Copy MathML from OneNote Equation
After edited equation completely, click "Design" tab -> "Gem" group -> "MathML" command.
"Gem" will pop up a "Equation MathML" window.
You can copy or modify MathML in this window.
"OK" button can save and convert MathML to OneNote equation.
Currently, only Firefox supports MathML.
Open the mathml.html by Firefox.