What's the source of this noise in my Arduino oscillator?
Try clipping your ground lead onto the ground pin that's physically closest to the pin you're looking at. If that doesn't change anything, and if you can, take up any slack in the ground lead of your scope by wrapping it around the probe.

I can't find a good article right off the bat (I'm sure that every scope maker out there has one) -- here's the few pointers I know:


Always ground the probe.  If the signal is the least bit fast or important, ground each probe close to the signal.
You can do the ground-wrapping trick to make things a bit better.
If it's a really sharp-edged signal and you need to be sure, ground the probe right at the tip.  If you pull the spring hook off the end of a typical O-scope probe, you'll see a metal sleeve around the tip.  That's the probe ground.  You can wrap a wire around that and ground it as close as possible to the signal you're probing (or you can look in the stuff that came with the probe and see if there's a little sprig-looking thing in there -- that's what it's for).
Make sure your probes are compensated, and calibrated if accuracy is important.


(If someone reading this knows a good article or YouTube presentation, please post a link -- this is one of those things that I've collected bits &amp; pieces of over the years but have never had to articulate clearly).