A320 flight crew reported oil loss on number one engine requiring diversion.

Narrative

On Day 0 Flight ABC ZZZ to ZZZ1 was operated. During the Captains cockpit preparation flow it was verified that the number one and number two engines had 22 quarts of oil. During the logbook review the Captain and First Officer were aware that the number one engine was being monitored for high oil consumption. During the preflight inspection; the First Officer thoroughly inspected with a flashlight; the interior and exterior of both the number one and number two engines and observed no evidence of oil leaking. The flight taxied out and departed for ZZZ1. Approximately 35 minutes in route the First Officer and Captain recognized the oil quantities were substantially different. The number one engine was indicating 11 1/2 quarts of oil. We begin monitoring the oil quantity level in the number one engine and determined that the engine was losing a half a quart of oil every four minutes. The Captain made a sat phone call to Dispatch and was patched in with Maintenance Control. Both were made aware of the situation and it was determined the flight would not be able to continue to ZZZ1. Dispatch and Maintenance coordinated the decision to go to ZZZ2 as our diversion city. The Captain informed the in-flight crew of the diversion to ZZZ2 and the possibility of shutting down the number one engine. The Captain then made a PA to the customers informing them of our situation and diversion to ZZZ2. There was no code issued at that time. There were no ECAM (Electronic Centralized Aircraft Monitor) messages or abnormal indications; we only had an advisory message when the oil quantity on the number one engine reached 5 quarts. During the descent passing through 22;000 feet the Captain started the APU in preparation for a potential engine shutdown. During the arrival phase into ZZZ2; priority handling was issued by ZZZ Approach on behalf of flight ABC. ARFF (Airport Rescue and Firefighting) equipment was dispatched and awaiting the arrival of flight ABC in ZZZ2. After safely arriving in ZZZ2; the number one engine was indicating one and a half quarts of oil prior to engine shut down at the gate. Once the engine was completely shut down it then indicated 7 quarts. Maintenance met the aircraft at the gate and advised the Captain and First Officer that he observed zero oil in the sight glass.