A panoramic scanning X-ray. An OPG relies on tomography i.e. images of specific radiographic planes are taken to make up the larger panoramic image where the maxilla and mandible are in the focal trough and the structures, superficial and deep to the trough, are blurred.
OPTs are used by health care professionals to provide information on:

Impacted wisdom teeth diagnosis and treatment planning - the most common use is to determine the status of wisdom teeth and trauma to the jaws.
Periodontal bone loss and periapical involvement.
Finding the source of dental pain
Assessment for the placement of dental implants
Orthodontic assessment. pre and post operative
Diagnosis of developmental anomalies such as cherubism, cleido cranial dysplasia
Carcinoma in relation to the jaws
Temporomandibular joint dysfunctions and ankylosis.
Diagnosis of osteosarcoma, ameloblastoma, renal osteodystrophy affecting jaws and hypophosphatemia.
Diagnosis, and pre- and post-surgical assessment of oral and maxillofacial trauma, e.g. dentoalveolar fractures and mandibular fractures.
Salivary stones (Sialolithiasis).
Other diagnostic and treatment applications.