Odoo's policy can be seen as to keep current Odoo version "as is", fixes mostly go in the next version. The OCA backports some of them in the OCB repository.

Odoo maintains its versions for only 4 years, encouraging users to jump in new versions every year. OCB accept fixes and backports for a longer time. OCB's versions receive patches and fixes as long as someone can dedicate work on it. Have a look on commits history for each version to see if it is maintained.

Adding something

Proposed PR must not create incompatibilities with Odoo (defined as: does a change allow the creation of community modules that are compatible with OCB but not with Odoo or the other way around).

References:

• 2016 The future of OCB on OCA mailing list. See these emails:
  1. https://odoo-community.org/groups/contributors-15/contributors-45694
  2. https://odoo-community.org/groups/contributors-15/contributors-45695
  3. https://odoo-community.org/groups/contributors-15/contributors-45710
  4. https://odoo-community.org/groups/contributors-15/contributors-45949
  5. https://odoo-community.org/groups/contributors-15/contributors-46014
  6. https://odoo-community.org/groups/contributors-15/contributors-46104
  7. https://odoo-community.org/groups/contributors-15/contributors-46153
  8. https://odoo-community.org/groups/contributors-15/contributors-46946
  9. currently, OCB doesn't accept backport of changes that could affects the migration procedure
  10. https://odoo-community.org/groups/contributors-15/contributors-46959
• 2014 Odoo v8 vs. saas-6 vs. OCB v8: What's the difference? on Odoo forum, for the time Odoo wasn't very stable