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. Polyamines are essential for cell proliferation and are implicated in cellular processes, ranging from DNA replication to apoptosis. H(+) + L-ornithine = CO2 + putrescine Inhibited by antizymes (AZs) OAZ1, OAZ2 and OAZ3 in response to polyamine levels. AZs inhibit the assembly of the functional homodimer by binding to ODC monomers. Additionally, OAZ1 targets ODC monomers 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 (PubMed:8106349, PubMed:10378276). Does not form a heterodimer with AZIN2 (PubMed:24967154). Expressed during testis development in the outer part of the seminiferous tubules. Belongs to the Orn/Lys/Arg decarboxylase class-II family.