Like Alycia, I had trouble finding a single source on police dissent. Below, I have a bunch of articles and some academic texts that I hope are helpful.
In addition to the examples that Alycia provided, there’s also a famous and recent incident with a retired Philadelphia police chief Ray Lewis protesting at OWS:
This is an article that interviews Lewis, and documents one other Oakland police officer on the record as supporting OWS:
These were the only references I could find to police joining OWS protests, but I thought I’d also point you to an article about Michael Moore calling for police to join OWS
In terms of what I could find about police and protesting, there are two main strains - whiste blowing on police corruption and protesting against working conditions.
About the former, this is a legal article on what makes whistle blowing difficult in police culture and why it’s necessary:
Here’s another academic article on obstacles to whistle blowing among police:
And here are some news articles on different instances of whistle blowing in various police departments: