"In order to" and "so as to" are used to express purpose and generally interchangeable. They are compound prepositions that have an infinitive as its object.
They are not always interchangeable. Both are prepositions, but "in order to" always has an infinitive as its object. "To" can be followed by an infinitive or a noun.
"In order to" is a compound preposition that always has an infinitive as its object. It's not unusual for an infinitive to be the object of a preposition.