Dynamic network slicing involving multi-operator network resources is challenging due to lack of cooperation and inter-operability. Currently available slicing techniques focusing on static network configurations without run-time modifications and prioritization are insufficient to provide on-demand, instantaneous network slicing. With currently developed open-source platforms for virtualization and orchestration such as ONAP and ORAN, dynamic network slicing can be enabled. In this paper, a novel dynamic resource allocation scheme for dynamic 5G network slicing (DYSOLVE) is proposed and evaluated for a vehicular emergency scenario. DYSOLVE allocates both the radio as well as transport network resources of multiple network operators cooperatively and aims for slice cost optimization while ensuring service availability and QoS. Our performance evaluation shows significant improvements in the proposed DYSOLVE scheme compared to a traditional baseline approach with fixed resource allocation and without multi-operator cooperation.