Mutations of the endoplasmic reticulum (ER)-stress transducer OASIS (encoded by CREB3L1), cause severe recessive osteogenesis imperfecta (OI) not compatible with surviving the neonatal period, as has been shown in two unrelated families through a whole gene deletion vs. a qualitative alteration of OASIS. Heterozygous carriers in the described families have exhibited a mild phenotype. OASIS is a transcription factor highly expressed in osteoblasts, and OASIS−/− mice exhibit severe osteopenia and spontaneous fractures. Here, we expand the clinical spectrum by a detailed phenotypic characterization of the first case of OASIS-associated OI surviving the neonatal period, with heterozygous family members being unaffected. All OI-associated genes were sequenced. Primary human osteoblast-like cell (hOB) and fibroblast (FB) cultures were obtained for qPCR, and steady-state collagen biochemistry. FB, hOB and skin biopsies were ultrastructurally analyzed. Bone was analyzed by μCT, histomorphometry, quantitative backscattered electron imaging (qBEI), and Raman microspectroscopy. The proband, a boy with severe OI, had blue sclera and tooth agenesis. A homozygous CREB3L1 stop codon mutation was detected by sequencing, while several family members were heterozygotes. Markedly low levels of CREB3L1 mRNA were confirmed by qPCR in hOBs (16%) and FB (21%); however, collagen I levels were only reduced in hOBs (5–10%). Electron microscopy of hOBs showed pronounced alterations, with numerous myelin figures and diminished RER vs. normal ultrastructure of FB. Bone histomorphometry and qBEI were similar to collagen I OI, with low trabecular thickness and mineral apposition rate, and increased bone matrix mineralization. Raman microspectroscopy revealed low level of glycosaminoglycans. Clinical response to life-long bisphosphonate treatment was as expected in severe OI with steadily increasing bone mineral density, but despite this the boy suffered repeated childhood fractures. Deficiency of OASIS can cause severe OI compatible with surviving the neonatal period. A marked decrease of collagen type I transcription was noted in bone tissue, but not in skin, and ultrastructure of hOBs was pathological. Results also suggested OASIS involvement in glycosaminoglycan secretion in bone. •CREB3L1 stop-codon causes OI type III phenotype similar to classical collagen I OI.•Heterozygous CREB3L1 stop-codon carriers without clinical phenotype.•OASIS-associated OI clinical phenotype widened and bisphosphonate effect as expected.•Tissue-specific effects, such as affected collagen I-transcription in bone, not skin•Homozygous CREB3L1 stop-codon associates with low levels of bone glycosaminoglycans.