Catalyzes the condensation of octanoyl-CoA, obtained from exogenously supplied fatty acids via beta-oxidation, with malonyl-[acyl-carrier protein], forming 3-oxodecanoyl-[acyl-carrier protein], an intermediate of the fatty acid elongation cycle that can then be extended to supply all of the cellular fatty acid needs. The enzyme thereby shunts fatty acid degradation intermediates from the beta-oxidation pathway into de novo fatty acid biosynthesis. H(+) + malonyl-[ACP] + octanoyl-CoA = 3-oxodecanoyl-[ACP] + CO2 + CoA Lipid metabolism; fatty acid biosynthesis. Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.