Turkey ovomucoid third domain (OMTKY3) is a canonical inhibitor of serine proteinases. Upon complex formation, the inhibitors fully exposed P1 residue becomes fully buried in the preformed cavity of the enzyme. All 20 P1 variants of OMTKY3 have been obtained by recombinant DNA technology and their equilibrium association constants have been measured with six serine proteinases. To rationalize the trends observed in this data set, high resolution crystal structures have been determined for OMTKY3 P1 variants in complex with the bacterial serine proteinase, Streptomyces griseus proteinase B (SGPB). Four high resolution complex structures are being reported in this paper; the three β-branched variants, Ile18I, Val18I, and Thr18I, determined to 2.1, 1.6, and 1.7 Å resolution, respectively, and the structure of the Ser18I variant complex, determined to 1.9 Å resolution. Models of the Cys18I, Hse18I, and Ape18I variant complexes are also discussed. The β-branched side chains are not complementary to the shape of the S1 binding pocket in SGPB, in contrast to that of the wild-type γ-branched P1 residue for OMTKY3, Leu18I. χ1 angles of approximately 40° are imposed on the side chains of Ile18I, Val18I, and Thr18I within the S1 pocket. Dihedral angles of +60°, −60°, or 180° are more commonly observed but 40° is not unfavorable for the β-branched side chains. Thr18I Oγ1 also forms a hydrogen bond with Ser195 Oγ in this orientation. The Ser18I side chain adopts two alternate conformations within the S1 pocket of SGPB, suggesting that the side chain is not stable in either conformation.