MongoDB - Replication Oplog Window has gone below 1 hours
If replication OpLog window goes under one hour, it means that timestamp between first and last  lines in the OpLog is under one hour. This can happen when there is lots of changes in the DB and your size of OpLog is "too small".
No, this is not fatal. It just means that your secondary nodes cannot "fallback" more than what is your OpLog window. I mean that if your window is f.ex. 55 minutes, you cannot stop your secondary node for over 55 minutes, because it cannot "catch-up" anymore and it needs to do "full sync" and if that full sync takes more than 55 minutes, it cannot be done.

What you can do is change OpLog size to bigger. If you double your OpLog size, you get double size window.