Forms pores that allow passive diffusion of small molecules across the outer membrane, including some antibiotics. Variation of the residues in the constriction zone modifies the transverse electric field in the zone, altering antibiotic resistance. (Microbial infection) Is not susceptible to CdiA-EC536-mediated toxicity, which uses OmpC-OmpF heterotrimers of some strains as its outer membrane receptor. Mutagenesis of extracellular loops L4 or L5 of this protein confers susceptibility to the toxin. Homotrimer (PubMed:22053181). Forms mixed heterotrimers with OmpF; other mixed heterotrimers are also probable (Probable). The N- and C-termini are two parts of the same strand. Extracellular loop 3 folds back into the lumen of the barrel forming a constriction zone that controls the pore size, while the trimer interface is formed by the packing of hydrophobic residues on the outer edges of beta strands 1 to 5 and further stabilized by extracellular loop 2 which reaches into the neighboring monomer (PubMed:22053181). Belongs to the Gram-negative porin family.