Organisationel level:
It is the lowest level of the netwrok : this level is private and belongs to an organization (companies, organization ...). 

OSPF or Open Shortest Path First:
Link-state, hierarchical IGP routing algorithm proposed as a successor to RIP in the Internet community. OSPF features include least-cost routing, multipath routing, and load balancing. OSPF was derived from an early version of the ISIS protocol.
 

OSI or Open System Interconnection:
International standardization program created by ISO and ITU-T (International Telecommunication Union Telecommunication Standardization Sector) to develop standards for data networking.