MongoDBインデックスと$or演算子

はい、$orクエリは必要に応じてインデックスを使用します。例：

> db.test.ensureIndex({a:1})
> db.test.ensureIndex({b:1})
> db.test.find({$or:[{a:1}, {b:2}]}).explain()
{
        "clauses" : [
                {
                        "cursor" : "BtreeCursor a_1",
                        "nscanned" : 0,
                        "nscannedObjects" : 0,
                        "n" : 0,
                        "millis" : 0,
                        "nYields" : 0,
                        "nChunkSkips" : 0,
                        "isMultiKey" : false,
                        "indexOnly" : false,
                        "indexBounds" : {
                                "a" : [
                                        [
                                                1,
                                                1
                                        ]
                                ]
                        }
                },
                {
                        "cursor" : "BtreeCursor b_1",
                        "nscanned" : 0,
                        "nscannedObjects" : 0,
                        "n" : 0,
                        "millis" : 1,
                        "nYields" : 0,
                        "nChunkSkips" : 0,
                        "isMultiKey" : false,
                        "indexOnly" : false,
                        "indexBounds" : {
                                "b" : [
                                        [
                                                2,
                                                2
                                        ]
                                ]
                        }
                }
        ],
        "nscanned" : 0,
        "nscannedObjects" : 0,
        "n" : 0,
        "millis" : 1
}