  The 6G Massive Multiple-Input Multiple-Output (MMIMO) networks can follow the
so-called User-Centric Cell-Free (UCCF) architecture, where a single user is
served by multiple Access Points (APs) coordinated by the Central Processing
Unit (CPU). In this paper, we propose how O-RAN functionalities, i.e.,
rApp-xApp pair, can be used for energy-efficient Serving Cluster Formulation
(SCF). Simulation studies show up to 37\% gain in Energy Efficiency (EE) of the
proposed solution over the state-of-the-art Network-Centric (NC) designs.
