MongoDB Ops Manager backup and restore The documentation claims that snapshots taken by ops manager are delta (differential) between successive snapshots. 


How do you then restore?
I mean if you are able to delete some snapshots according to
retention, how do you connect the dots in between to restore to a
point in time.
What do they mean by base snapshot? Is that whole backup?
How can a given snapshot become full and ready for restore if any
snapshot before it is deleted?


I understand one can replay oplog to go forward to a point in time from a given snapshot. This question is if any snapshot before the snapshot where restore begins gets deleted how can you restore?

From MongoDB docs:

"...The snapshot only stores the differences between successive snapshots. Snapshots use only a fraction of the disk space required for full snapshots."

Any help is much appreciated.
I'm not sure which version of Ops Manager you were running, but there's been a lot of improvements in the last year. The latest docs explain the space-spacing approaches of the different storage choices:


MongoDB blockstore and AWS S3 bucket  - Each successive snapshot stores only the differences from the previous snapshot. Compression and block-level de-duplication reduce the size of snapshot data.  
File system store - uses compression only.