Old fashioned oats are steamed then rolled (aka flaked). Quick oats are flaked oats cut into pieces to create more surface area to cook faster. You shouldn’t need a cereal mash for either.
There could be a number of reasons behind why you did not hit your planned OG. The expired oats could be the reason, or a bit of six row could have helped, or dough balls, or any other number of contributing factors attributed to process, technique, or the fact that you’re dealing with agricultural products that have inherent variability. We tend to expect the raw material to be consistent yr to yr, batch to batch but it simply isn’t.
For example, I recently had the opposite problem. I overshot OG by a metric mile. I had to add RO water to dilute the gravity. While Amber and lighter have been spot on, Dark beers have tended to end up high in my brewery lately. Like you, the important thing is I took measurements, recognized the problem, and applied a simple fix.
To account for variability, I allow close enough to be good enough. A point or three here or there is acceptable to me. Occasionally, I end up outside close enough tolerance so adjustments have to be made sometimes planned, sometimes on the fly. I have DME as well as several types of sugar on standby. Fortunately, I don’t have to use them often.