Oral cholecystogram: Abbreviated OCG. An x-ray procedure for diagnosing gallstones. The patient takes iodine-containing tablets by mouth for one night or two nights in a row. The iodine is absorbed from the intestine into the bloodstream, removed from the blood by the liver, and excreted by the liver into the bile. The iodine, together with the bile, is highly concentrated in the gallbladder. Iodine is used in an OCG because it is dense and radioopaque (stops x-rays). It outlines the gallstones that are radiolucent (x-rays pass through them) and that are usually invisible on x-ray. The failure to visualize the gallbladder on an OCG is evidence for a diseased gallbladder. The OCG is an excellent procedure for diagnosing gallstones when they are strongly suspected but cannot be seen by ultrasound.