There is much more to learn about developing apps using OSGi. Several resources are listed below and many more abound. To make the best of your time, however, avoid OSGi service articles that explain techniques that are older and more complicated than Declarative Services.
Developers new to OSGi should check out these resources:
Introduction to Liferay Development: For using OSGi to develop on Liferay Portal.
OSGi Alliance’s Developer section explains OSGi’s architecture and modularity.
If you’re ready to dive deep into OSGi, read the OSGi specifications. They’re well-written and provide comprehensive details on all that OSGi offers. The OSGi Alliance OSGi Compendium: Release 6 specifies the following services that Liferay Portal CE 7.1 leverages extensively.
Declarative Services Specification
Configuration Admin Service Specification: For modifying deployed bundles. Since Configuration Admin services are already integrated with Declarative Services, however, Liferay developers need not use the low-level API.
Metatype Service Specification: For describing attribute types as metadata.