Assembly of the E. coli bacteriophage P2 into an icosahedral capsid with T = 7 symmetry is dependent on the gpN capsid protein, the gpQ connector protein and the gpO internal scaffolding protein. In the presence of the P4-encoded protein Sid, the same proteins are assembled into a smaller capsid with T = 4 symmetry. Although gpO has long been expected to act as an internal scaffolding protein, it has not been possible to produce P2 procapsids efficiently in vitro or in vivo due to a failure to express gpO at high levels. In this study, we find that full-length gpO undergoes proteolytic degradation within 1 h of induction of expression. However, a truncated version of gpO lacking the N-terminal 25 amino acids (Odelta25) is stably expressed at high levels and is able to direct the formation of P2 size procapsids. In the presence of Sid, Odelta25 is incorporated into P4 procapsids, showing that Sid overrides the effect of gpO on capsid size determination.