With the advent of 6G, communication networks face unprecedented challenges, particularly in the realms of comprehensive scenario representation and knowledge inference. Founded on the concept of Channel Knowledge Map (CKM), this paper establishes the 6G omni-scenario knowledge base for on-demand service (OSOD 6G). Specifically, we first construct the ontology-based framework for 6G space-air-ground-sea scenarios, which elucidates the intrinsic relationships among subject, environment, requirement, and knowledge, thereby providing a solid foundation for effective 6G knowledge representation. Secondly, we deploy diverse signal sources and receivers both aerially and terrestrially in accordance with various scenarios. Meanwhile, CKM is employed to extract channel knowledge, thereby enabling the inference of channel characteristics and the construction of OSOD 6G. Furthermore, we conduct visualization and simulation verification of OSOD 6G from multiple dimensions of omni-scenario knowledge inference, including diverse signal transmitters, urban typologies, receiver altitudes, and CKM fitting performance. Finally, two representative use cases are presented to demonstrate the scenario-awareness and on-demand service capabilities of OSOD 6G.