Background: Diagnosis and management of congenital anomalies of the spine can be a challenge because of their complex presentations. We present an uncommon case of congenital deformity of the spine with L4-L5 vertebral fusion, mimicking a single vertebra, and L3 spondylolisthesis treated by oblique lumbar interbody fusion (OLIF). Case Description: A 69-year-old woman presented with increasing lower back pain radiating to the left leg, with aggravation of symptoms for the past 6 months, causing difficulty in walking. She also complained of paresthesia along the L3-L5 dermatomes in both legs, with more prominence on the left side. Imaging revealed fusion deformity of the L4-L5 vertebrae, as well as degenerative spondylolisthesis at the L3-L4 level. After treatment with OLIF, the patient had an uneventful recovery period. Comparisons were made between the preoperative and 6-month follow-up visual analog scale and the Oswestry Disability Index scores. The patient showed significant improvement in the scores, as well as in her symptoms. Conclusions: OLIF is a promising technique that can be applied in the management of degenerative disk diseases and also for deformities that may be formidable to treat by adopting the traditional posterior approach. ? 2019 Elsevier Inc.