In the following sentence, should "Who" or "Whom" be used in the blank space?: To ___ do I owe my thanks?
To whom ...
'Whom' is employed when it is the object of the sentence, 'who' if the subject.
A sentence is usually made with a subject, a verb, and an object. The sentence,"John buys eggs" , has the subject ' John', the verb 'buys' and the object 'eggs'. Your sentence, rearranged, becomes "I owe my thanks to .......", and the blank position is for the object.
When in the subject position, 'who' is used. "Who buys eggs?"