While 'Oh, I see.' expresses a realization, it is structured as a positive statement, making it an affirmative sentence. It doesn't ask a question, give a command, express strong emotion with an exclamation mark, or contain a negative element as defined for the other types.
Class 6 › English Grammar and Composition › Chapter 6: Sentences › Topic: Definition of a Sentence
Aligned to the NCTB national curriculum.