You might be able to define the 'math' word via a custom dictionary in OneNote.
From MS Office article "Add words to your spell check dictionary":
When you use the spelling checker, it compares the words in your document with those in the main dictionary. The main dictionary contains most common words, but it might not include proper names, technical terms, or acronyms. In addition, some words might be capitalized differently in the main dictionary than in your file. Adding such words or capitalization to a custom dictionary prevents the spelling checker from flagging them as mistakes.
- Click the File tab.
- Under Help, click Options.
- Click Proofing.
- Make sure the Suggest from main dictionary only check box is cleared.
- Click Custom Dictionaries.
- In the Custom Dictionaries dialog box, make sure the check box next to each custom dictionary that you want to use is selected.
Note: I have not tried this myself, so I'm only surmising that this will also affect the OCR. :) You may want to dig around the linked page(s) a bit to figure out how to make the custom dictionary (unless you're already familiar).