I have seen several references made by Fabien Pinakers, CEO of Odoo, that approximately 75% of Odoo Code is developed in the community, and by implication Open Source, rather than by the company and sometimes Closed Source. 

Finding statistics to back this up is a complicated matter, but looking at the Odoo App Store can provide a good start.  https://apps.odoo.com/apps.  Clicking "All Apps" will give you a count - current just now standing at 43,943 distinct "apps" or "modules" which are Not part of Odoo's core Community and Enterprise package.

That is a lot of wheat to thresh, test, and find your way through.  Of that count 18,502 are showing as "open source" (under the price drop down) while 25,441 are showing today as "paid" apps.  By Apps we mean "modules" which can be added to the standard Odoo 

Apps are critical to implementing effective Odoo builds for nearly any business.  It is nice to think about running a business with just the core modules, and, if you go with the Enterprise implementation ($30-$40 per month per user), then you can get important functionality.

If you are going with the Odoo Community implementation you will definitely need to update it.  For instance, if you want to add your preferred payment processor, or if you want to have shipping integration from FedEx, UPS etc.

There are so many ways you can, and normally should modify (using 3rd party built modules), and even using the built in Odoo "Master Apps" - top level subjects,  you need to choose, test and decide what features you want, what you don't, all with the goal of making you and your team most efficient in running your business.

Enter the OCA, Odoo Community Association and where I like to start in any module evaluation and quest.

The OCA, or Odoo Community Association is an organization dedicated to developing and maintaining functional and critical Odoo modules which make running Odoo effective, and normally more efficient and capable.  There are at present 232 OCA Repositories hosted on Github, covering OpenERP/Odoo branch builds and branches for the multiple Odoo version generations. https://github.com/orgs/OCA/repositories

What makes the OCA so valuable is their strong commitment to community review and testing.  There is full visibility in OCA modules of changes made through time.  There is an easy method for testing, via OCA's Runboat.  There is a possibility and open invitation to participate.

What there is not, however I feel, is a consistent commitment to making clear documentation, nor is there a clear path for finding support in the community or even from the developer community.  

And yes, there is a very wide community at OCA none the less, with tremendous participation within it.

It is a goal here, that I can bring some light, at least to my process, of making sense and making use of Modules that are available, of looking at ways to participate and contribute, and ultimately, of helping to show the way on how to use the very capable and expansive set of Apps and Modules that are available with Odoo, the OCA, and also with the wider realm of both Open Source, and Custom and "Less Open" source Odoo customizations.

I look forward to building this out, and also bring some of my past notations into this space.