Inclusion criteria:  Diagnostic confirmation during screening period of spinal muscular atrophy (SMA) caused by biallelic pathogenic variants affecting survival motor neuron 1 (SMN1) and 2-4 copies of survival motor neuron 2 (SMN2) Naive to survival motor neuron (SMN) dependent treatments (e.g., risdiplam and nusinersen). Onset of clinical signs and symptoms at ≥ 6 months of age A complete Hammersmith Functional Motor Scale - Expanded (HFMSE) assessment during the screening period for trial eligibility Able to sit independently at screening, but has never had the ability to walk independently.  Exclusion criteria:  Anti-adeno-associated virus serotype 9 (AAV9) antibody titers > 1:50 at screening as determined by sponsor designated central lab. NOTE: A negative anti-AAV9 antibody titer is defined as ≤ 1:50. Infectious process (e.g. viral, bacterial) or febrile illness within two weeks prior to start of screening, and up to OAV101 treatment or sham procedure Hepatic dysfunction (i.e., aspartate aminotransferase (AST), alanine aminotransferase (ALT), bilirubin, gamma-glutamyl transferase (GGT) or glutamate dehydrogenase (GLDH), > upper limit of normal (ULN). Requiring invasive ventilatory support, or awake noninvasive ventilatory support for > 6 hours during a 24-hour period, or noninvasive ventilatory support for > 12 hours during a 24-hour period. Complications at screening, as determined by the Investigator, that would impact the participant's ability to perform the motor efficacy assessments.