Any thoughts on how to get LaTeX in OneNote?
[Not quite the answer you are expecting]
No, there's no way to include LaTeX (and I assume you mean the mathematics typeset with it) in OneNote.
One workaround that is sort-of-kinda-working (with severe caveats), so go ahead, if you absolutely need to. This applies to OneNote 2007, so if you have more recent version, it could be somewhat different.
Now, the caveats:
That being said, if you'd like to have a note-taking application together with LaTeX mathematics, I suggest turning to a wiki software that supports advanced typeset mathematics. I personally use Confluence, but free alternatives are available (the example that first comes to mind is MediaWiki, which powers Wikipedia, and it has comprehensive LaTeX support).
You will need to set up and administer a web server (and most likely a database server as well) in order to use wiki software, but in the long term, and especially if you are serious about having a space to collect your stuff, this is a superior solution.
In OneNote 2010, at least (which is the only version I have), you can enter at least some LaTeX directly in math mode. I say at least some because I am discovering it by experimentation rather than reading about it in OneNote documentation (if any such documentation exists).
Here are a few examples (after first selecting the type tool and entering math mode via cmd-=):
The trick with actually getting it to typeset is to hit the space bar at certain places. For instance, above, I'd hit the space bar after the
There is a lot more to discover (and I am only showing a few things I have found, but I am very pleased with what I have found so far. Oh, one other item that is helpful is that you can get textbook style fraction formatting and paren or bracket resizing by hitting space after them.
then type your equation and hit the Enter key. Right click on the automatically generated image and copy it into my OneNote file.
To expand on Joe's answer, while there is no support for actual LaTeX, the LaTeX like language Joe describes is very powerful and convenient. It supports most things you'd want: matrices, subscripts, superscripts, blackboard bold, etc. The only thing I haven't been able to do are commutative diagrams with too many arrows.
Try typing this while editing text (what you'll see happening might feel confusing the first time):
It is pretty quick to learn, but does suffer from very obscure documentation and configuration. The best documentation I found is the pdf titled "Unicode Nearly Plain-Text Encoding of Mathematics" (linked to from a somewhat helpful OneNote help page). The name Microsoft uses is "Math AutoCorrect", and the (limited) configuration is in Options -> Proofing -> AutoCorrect Options -> Math AutoCorrect tab.
Unfortunately, this type of equations is not at all supported by the online, Android or iOS OneNote.