B737 First Officer reported loss of engine oil quantity during cruise. Flight crew diverted and after landing found evidence of oil leak.

Narrative

During cruise Captain did a check of the of the secondary ENG (Engine) instruments and noticed the oil quantity on ENG 1 was significantly lower (6) then ENG 2 (19). After discussion we decided to Monitor; get the checklist ready and begin looking for possible diversion airports. We also sent a message of Dispatch to keep them informed. Shortly thereafter the quantity dropped to 5; the reverse coloring appeared on the display and we began to notice a fluctuation of about + - 10 PSI of the oil pressure. We [notify ATC of our situation] and requested to vectors to ZZZ. I began to read the Low Oil Quantity Checklist but had to discontinue it due to our close proximity to ZZZ and the amount of tasks left to handle. The quantity dropped to 4 just a minute or two after and the Captain asked me to [request priority handling]. We were given radar vectors to ZZZ. The Captain continued to fly and communicate with FAs (Flight Attendant) and PAX (Passengers) while I handled ATC; Dispatch and the box. Dispatch was able to send us a release and weather for ZZZ. All normal checklists were completed on schedule. As we were below 10000ft; the quantity dropped to 1qt. I elected to have the ENG Shutdown checklist up on my iPad ready if we have to shutdown. We also started the APU. We quickly discussed the possibility of delay vectors if we had to shutdown but continued towards the airport. At some point below 1000ft the oil quantity blanked out. We touched down shortly after uneventfully and ARFF (Airport Rescue and Firefighting) was waiting for us. Once we exited the runway they approached the aicraft and confirmed oil streaks along ENG 1. We shutdown the ENG immediately upon confirmation. We taxied to Gate XX and accomplished normal shutdown procedures. After shut we noticed that Oil quantity on ENG 1 increased to 3qts. The difficulty with an unplanned diversion and lack of time cannot be overlooked. It was difficult to accomplish all tasks in a short amount of time. It also felt like we were racing against the oil quantity. I feel that one thing we could have done to help the situation was reduce ENG 1 to idle. That may have bought us some more time and reduced the racing" or "running out of time" feelings I'm sure we were both feeling. The idea had come through my mind but I didn't speak up as we were both heavily task saturated and I did not want to stress the Captain any more."