Narrative:

We were flying to ZZZZ at FL330. We had just crossed over the zzzzz intersection and therefore from ZZZ center to ZZZZ control. The first officer noticed the engine display white and boxed. It was clear the number one engine was losing oil quickly. It was down to 18 percent and quickly fell to 16 percent and trending downwards. We had left with 98 percent. An engine failure appeared to be imminent. The QRH was referenced. Low oil quantity is not referenced in the QRH. We were prepared to run the high oil temp or low oil pressure checklist when needed. I [advised] ATC and requested lower and a 180 degree turn towards ZZZZ1. A turn and lower was given by ZZZZ control and we were switched back to ZZZ ARTCC.a flaps 15 landing was decided upon in case the engine failed on approach. Dispatch; F/as; passengers and ZZZZ1 ops were all brought into the loop. Dispatch was contacted though arinc; but due to poor reception; an ACARS message was sent additionally to ensure communication. We landed on runway xy in ZZZZ. On approach; which was 15-20 minutes after oil leak discovery; the oil quantity was down to 10 percent. All other engine parameters indicated normally. A two engine approach with flaps 15 was successfully completed. Once at the gate; a visual inspection of the engine by both the first officer and me was performed. There was a large amount of oil leaking from the number one engine; validating our oil quantity gauge. Once on the ground; dispatch; the chief pilot on call; operations; and maintenance were are all spoken to. The passengers were kept in the loop.we continued to stay with the aircraft and assisted contract maintenance in an engine motor. The intent was to find where the leak was to help company bring the proper parts and tools. With the oil refilled to 100 percent and the engine motored for a short period the oil quickly drained from 100 percent to 88 percent and poured out of the accessory gear box.

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Original NASA ASRS Text

Title: B737 flight crew reported an oil leak in flight that required them to divert.

Narrative: We were flying to ZZZZ at FL330. We had just crossed over the ZZZZZ intersection and therefore from ZZZ Center to ZZZZ Control. The First Officer noticed the engine display white and boxed. It was clear the number one engine was losing oil quickly. It was down to 18 percent and quickly fell to 16 percent and trending downwards. We had left with 98 percent. An engine failure appeared to be imminent. The QRH was referenced. Low oil quantity is not referenced in the QRH. We were prepared to run the High Oil Temp or Low Oil Pressure Checklist when needed. I [advised] ATC and requested lower and a 180 degree turn towards ZZZZ1. A turn and lower was given by ZZZZ Control and we were switched back to ZZZ ARTCC.A flaps 15 landing was decided upon in case the engine failed on approach. Dispatch; F/As; Passengers and ZZZZ1 ops were all brought into the loop. Dispatch was contacted though ARINC; but due to poor reception; an ACARS message was sent additionally to ensure communication. We landed on Runway XY in ZZZZ. On approach; which was 15-20 minutes after oil leak discovery; the oil quantity was down to 10 percent. All other engine parameters indicated normally. A two engine approach with flaps 15 was successfully completed. Once at the gate; a visual inspection of the engine by both the First Officer and me was performed. There was a large amount of oil leaking from the number one engine; validating our oil quantity gauge. Once on the ground; Dispatch; the Chief Pilot on Call; Operations; and Maintenance were are all spoken to. The passengers were kept in the loop.We continued to stay with the aircraft and assisted Contract Maintenance in an engine motor. The intent was to find where the leak was to help Company bring the proper parts and tools. With the oil refilled to 100 percent and the engine motored for a short period the oil quickly drained from 100 percent to 88 percent and poured out of the accessory gear box.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.