The gemara discusses whether a korban o'lah can atone for a mitzvas aseh that was "violated" (or not kept) after designating the animal for the korban. The gemara justifies the rationale that it would work, even though by a chatas it wouldn't work, is that an o'lah can atone for many violations whereas a chatas can only atone for one (each aveirah violated would demand a separate korban chatas). Rashi explains that the Torah doesn't require an independent korban olah for each aseh that was violated because it is "only a gift".
It seems to me that Rashi can be better explained based on the Ramaban in the aseres hadibros who writes that a positive mitzvah is a method of representing "love" whereas the lo ta'aseh in the torah is essentially yir'ah - fear. The idea behind mitzvas aseh is that we get up and do something to demonstrate our love and devotion to Hashem. There is no specific consequence or punishment for not doing, because then the doing could be out of fear, and the entire point of these mitzvos is to provide us with a method of showing love for Hashem. Therefore, it follows that a chatas which comes for the violation of an aseh is in place of the punishment that one deserves for the violation. Each aveira deserves it's own punishment and therefore demands it's own korban chatas. But, the concept of a korban olah atoning for a positive mitzva is that it provides us with a method of "donating" a gift to Hashem, thereby grabbing the opportunity to express our love for Hashem that we missed by forfeiting the opportunity to fulfill the aseh. It is for this reason that one Olah can atone for many mitzvos aseh because it is not mitzva specific, rather it is a method of showing our love and devotion for Hashem for the one or many opportunities that we missed to express our love.
This point is supported from the gemara on 7b and rashi which explains that an o'lah can only be brought on an aseh after teshuva has been done and the person technically already received atonement. The entire function of the olah is to serve as a gift. Rashi expresses this in context of a parable of one who committed a crime against the king and already appeased him, when he comes to greet the king he brings along a gift.