Atlas' Free Tier M0 clusters do not support querying the oplog with a namespace other than a string. As such, it may be worth updating the Atlas M0 (Free Tier) Limitations section of the Atlas documentation to adequately reflect this limitation.

Upon attempting to query the oplog with the $regex operator yields the following results:

free-shard-0:PRIMARY> db.oplog.rs.find( { ns: { $regex: ".*" } } )
Error: error: {
	"ok" : 0,
	"errmsg" : "querying oplog with an ns other than a string is not supported in this atlas tier",
	"code" : 8000,
	"codeName" : "AtlasError"
}