Here You probably have only one option: let the OC or BB run in subfolder/subdomain/different domain, etc. Do not integrate them as You are mentioning.
Instead of this create some own API (this would be the cleanest way) on the BB side and an API client on the other one (API will contain methods providing the required data).
The other solution is to implement new model(s) in OC (maybe by just copying from BB) that will directly read data from BB database so that You can call them in Your OC controllers and display the data the normal way OC does... This is also considered clean solution.