INTRODUCTION
The structure of Parshat Vayikra is:
Chapter 1 - Olah offerings
Chapter 2 - Mincha
Chapter 3 - Shelamim
Chapter 4 - Chatas
Chapter 5 - Oleh v'yored, Asham
Chapters 1-3 are voluntary offerings.
אדם כי יקריב מכם IF A MAN OF YOU OFFER [AN OFFERING] — This means, when he offers: Scripture is speaking here of voluntary offerings.
Rashi is compelled to say, based on the wording of the verse, that the verse speaks of voluntary offerings. However, this begs the question, why would the Torah begin with the laws of voluntary offerings?
Wouldn't it make more sense to begin with what we are obligated to do, and then continue to what we may volunteer to do?
He shall lay his hand upon the head of the burnt offering, that it may be acceptable in his behalf, in atonement for him.
ונרצה לו AND PROPITIATION SHALL BE EFFECTED FOR HIM — For what kind of sins does it (the עולת נדבה) effect propitiation for him? Should you say, “for such as make him liable to excision, or to execution by the court, or to the death penalty through the Heavenly Judge, or to lashes, surely, you see that the punishment for those sins is expressly stated and it is that and not the sacrifice which effects propitiation! Consequently it can only atone for the neglect of a positive command and for the transgression of “a prohibition transformed into a positive command” (לאו הניתק לעשה, a prohibitive law the transgression of which can be repaired by a succeeding act) (Sifra, Vayikra Dibbura d'Nedavah, Chapter 4 8).
Based on the above Rashi, perhaps we can say that an Olah offering -- while not technically an obligation -- is related to an obligation, because it can remove a person's liability for punishment.
However, if that was the case, then why would the Torah interject other voluntary offerings (i.e., Mincha and Shelamim) between the Olah and Chatas offerings?