A family of Microsoft products that enable users to capture, organize, and reuse notes electronically.
Jay,
I think that if you're using OneNote often, or planning on using OneNote often, that the notion of 'typing the equation' will soon become second nature, though it may take some time to get used to the syntax in the beginning. The OneNote typed syntax, shared with Word, isn't so far off from what you might think it would be.
For example, the following can be created:
The syntax is very similar to that used by LaTeX. Page 7 of this document from a web search gives the Office equation syntax you might be looking for, among other helpful shortcuts: http://www.iun.edu/~mathiho/useful/Equation%20Editor%20Shortcut%20Commands.pdf
If you're using it regularly, I also think that typing would go much faster than using the Word equation GUI to construct your equations.
Otherwise: no. It does not appear that OneNote contains an equation editor GUI.
Good luck!