Right, but the crux of OIG's finding is that there isn't a hard difference between the two.

The plane was not built to the spec that FAA type-certified. But at the same time it wasn't clearly out of certification either.

There were significant changes made to MCAS since the the type certification. However, there was no clear delineation of what types of post-certification changes would require another FAA type certification.

The permeability of the two types of certifications and the amount of design changes that can happen between them is the entire problem OIG is pointing out.

> FAA essentially accepted proofs / was pressured to accept assertions from Boeing that the type was actually safe

No, the document says Boeing's ODA team was pressured to accept assertions from Boeing's management. The Boeing ODA team is employed by Boeing. They are given de facto self-certification authority either by actually self-certifying (in the airworthiness case), or by choosing what it flags to FAA for external certification (in the design/type case).

No, an early, low-risk version of MCAS was proposed and given type approval by FAA. Then the MCAS design was changed iteratively and continuously without prompting another type approval process. Thenceforth, Boeing's ODA self-certification process was the only statutorily required step that could have caught that the post-type approval changes actually increased the risk significantly.

You should read the OIG report. It actually discusses all of this. It is absolutely possible an FAA cert would've missed the MCAS issue as well, but as OIG points out, one variable was significant commercial pressure on Boeing's ODA to approve the (iterated-since-type-approved)-MCAS. Presumably FAA staff would be less susceptible to this type of commercial pressure.

Design changes de facto are part of the self-cert process because it had no requirement for FAA oversight of post-flight design changes.

DOT OIG disagrees that this played no role. Here's from Page 2 of their report, i.e. the entire "Findings" summary:

While FAA and Boeing followed the established certification process for the 737 MAX 8, we identified limitations in FAA’s guidance and processes that impacted certification and led to a significant misunderstanding of the Maneuvering Characteristics Augmentation System (MCAS), the flight control software identified as contributing to the two accidents. First, FAA’s certification guidance does not adequately address integrating new technologies into existing aircraft models. Second, FAA did not have a complete understanding of Boeing’s safety assessments performed on MCAS until after the first accident. Communication gaps further hindered the effectiveness of the certification process. In addition, management and oversight weaknesses limit FAA’s ability to assess and mitigate risks with the Boeing ODA. For example, FAA has not yet implemented a risk-based approach to ODA oversight, and engineers in FAA’s Boeing oversight office continue to face challenges in balancing certification and oversight responsibilities. Moreover, the Boeing ODA process and structure do not ensure ODA personnel are adequately independent. While the Agency has taken steps to develop a risk-based oversight model and address concerns of undue pressure at the Boeing ODA, it is not clear that FAA’s current oversight structure and processes can effectively identify future high-risk safety concerns at the ODA.

https://www.oig.dot.gov/sites/default/files/FAA%20Certificat...