Is there a way to let users edit something only if they are a members of an OG?
You could, for sure, use the Group module to let users edit something if they are a member of some group. Group allows for creating arbitrary collections of your content and users on your site, and grant access control permissions on those collections. It is available as of D7, and has a D8 version also. It is designed to be an alternative to Organic Groups. The Group module creates groups as entities, making them fully fieldable, extensible and exportable.

For this specific question, you'd enable the gnode submodule, and for each group type you would define the appropriate permissions (view, edit, delete, etc) for the various Content Types. That's it.

For more details about how Group compares to Organic Groups, refer to my answer to "What are the features of the Group module versus Organic Group module?".

Resources


Community documentation for the Group module.
Video tutorial "How to use the Group module in Drupal 8 to manage editorial access control to different areas of a web site".