An expansion of services and unprecedented traffic growth is anticipated in future networks, aligned with the adoption of the
long-awaited Fifth Generation (5G) of mobile communications. To support this demand, without exposing mobile operators
to the pressure of CAPEX and OPEX, 5G uses new frequency bands, and adopts promising trends, including: densification,
softwarization, and autonomous management. While the first technology is proposed to handle the traffic growth requirements,
the softwarization and autonomous management are expected to play, in synergy, to ensure the desired trade-off between
reducing the CAPEX and OPEX, while guaranteeing the quality of service (QoS). Softwarization is expected to transform the
network design, from one size fits all, to more demand oriented adaptive resource allocation. In this work, we focus on this
point, by discussing how these technologies act in synergy towards enabling RAN sharing. Particularly, we focus on how they
fit into the issue of energy efficient Multi-Operator Resource Allocation (MO-RA). After a survey and classification of
schemes leveraging this synergy for distinct resource allocation (RA) objectives, we present a detailed survey and qualitative
classification of RA schemes with respect to energy efficiency. This work presents an innovative survey, since it concentrates
on multiple operators, and the enabling of Mobile Virtual Network Operators (MVNOs), which will come into play with the
complete virtualization of mobile networks. Based on the deep literature analysis of the different operations that can bring
energy savings to MO-RA, we conclude the work with listing open challenges and future research directions.