MongoDB 3.0 write lock on oplog collection in replica mode

In replica mode each write operation to any collection in any DB, also writes to the oplog collection.

Now, when writing to multiple DBs in parallel, all these write operations also write to the oplog. My question: do these write operations require locking the oplog ? (I'm using w:1 write concern). If they do, this is kind of similar to having a global lock between all the write operations to all the different DBs, isn't it ?

I'd be happy to get any hints on this.