Interferon-induced, dsRNA-activated antiviral enzyme which plays a critical role in cellular innate antiviral response. In addition, it may also play a role in other cellular processes such as apoptosis, cell growth, differentiation and gene regulation. Synthesizes preferentially dimers of 2'-5'-oligoadenylates (2-5A) from ATP which then bind to the inactive monomeric form of ribonuclease L (RNase L) leading to its dimerization and subsequent activation. Activation of RNase L leads to degradation of cellular as well as viral RNA, resulting in the inhibition of protein synthesis, thus terminating viral replication. Can mediate the antiviral effect via the classical RNase L-dependent pathway or an alternative antiviral pathway independent of RNase L. 3 ATP = 5'-triphosphoadenylyl-(2'->5')-adenylyl-(2'->5')-adenosine + 2 diphosphate Produced as a latent enzyme which is activated by dsRNA generated during the course of viral infection. Strongly activated by long dsRNAs at least 50 nucleotides in length. ssRNA does not activate the enzyme. Monomer. Intestine. By type I interferon (IFN) and viruses. OAS domain 3 is catalytically active. OAS domain 1 has no catalytic activity but is essential for recognition of long dsRNAs. Belongs to the 2-5A synthase family.