An applyOps oplog entry contains a list of individual ops to apply in its "o" field. Each of these ops should have a "tid" field, and the outer applyOps oplog entry should have a "tid" field as well, i.e. it should look something like:

{
op: "c",
"ns" : "<dbName>.$cmd",
"tid": <tenantId>,
"o" : {
"applyOps" : [
    {
    "op" : "i",
    "ns" : <dbName>.<coll>,
    "tid": <tenantId>,
    "o" : { ...},
    }
]
}

Change applyOps to take a DatabaseName obj, and then pass the DatabaseName object from the nss field to applyOps in applyApplyOpsOplogEntry, so that the DatabseName can be used to lock the db. Then, in _applyOps we should construct the NamespaceString for each individual op using the "nss" and "tid" fields.

When writing oplog entries for ops that occurred as part of a transaction, we log an applyOps entry upon either prepareTransaction or commitTransaction (for an unprepared transaction). When applying these operations during oplog application, we'll apply each op individually.