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. Displays antiviral activity against Chikungunya virus (CHIKV), Dengue virus, Sindbis virus (SINV) and Semliki forest virus (SFV). 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 (Probable). Strongly activated by long dsRNAs at least 50 nucleotides in length (PubMed:25775560). ssRNA does not activate the enzyme (PubMed:25775560). Monomer. Present at high level in placenta trophoblast. 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.