[{"FirstName":"abc","LastName":"xyz"},{"FirstName":"def","LastName":"ghi"},{"FirstName":"123","LastName":"789"},{"FirstName":"opq","LastName":"rst"},{"FirstName":"456","LastName":"654"}]

db.collection.find({$expr:{$or:[{$and:[{$eq:["$FirstName","abc"]},{$eq:["$LastName","xyz"]}]},{$and:[{$eq:["$FirstName","123"]},{$eq:["$LastName","789"]}]},{$and:[{$eq:["$FirstName","a1b2"]},{$eq:["$LastName","c7d8"]}]}]}})