Catalyzes the first and rate-limiting step of polyamine biosynthesis that converts ornithine into putrescine, which is the precursor for the polyamines, spermidine and spermine (PubMed:7498733, PubMed:19762559). Polyamines are essential for cell proliferation and are implicated in cellular processes, ranging from DNA replication to apoptosis (PubMed:19762559). H(+) + L-ornithine = CO2 + putrescine Inhibited by antizyme (AZ) in response to polyamine levels. AZ inhibits the assembly of the functional homodimer by binding to ODC monomers and targeting them for ubiquitin-independent proteolytic destruction by the 26S proteasome. Amine and polyamine biosynthesis; putrescine biosynthesis via L-ornithine pathway; putrescine from L-ornithine: step 1/1. Homodimer. Only the dimer is catalytically active, as the active sites are constructed of residues from both monomers. Deletion mutant pc13 has a reduced brood size and reduced enzymatic activity (PubMed:7498733). Smaller body size as compared to wild-type. Reduced putrescine and spermidine levels (PubMed:19762559). Double knockout with the polyamine transporter catp-5 results in a reduced brood size, delayed postembryonic development, and a more marked reduction in putrescine and spermidine levels as compared to the single mutants (PubMed:19762559). Belongs to the Orn/Lys/Arg decarboxylase class-II family.