DDOST-CDG, caused by biallelic pathogenic variants in DDOST which encodes dolichyl-diphospho-oligosaccharide-protein glycosyltransferase, a subunit of N-glycosylation oligosaccharyltransferase (OST) complex, is an ultra-rare condition that has been described in two patients only. The main clinical features in the two reported patients include profound developmental delay, failure to thrive, and hypotonia. In addition, both patients had abnormal transferrin glycosylation. Here we report an 18-year-old male who presented with moderate developmental delay, progressive opsoclonus, myoclonus, ataxia, tremor, and dystonia. […] Our study highlights the clinical variability, expands the clinical and biochemical phenotypes, and describes new genotype, which all are essential for diagnosing and managing patients with DDOST-CDG.