Was supposed to do an oral exam, but was instead presented with my grade
There is no ethical issue here if no one was disadvantaged by this. Accept that grading can be an inexact science even in the best situations. The professor made a judgment about the quality of your work overall and suggested a grade. His expectations, apparently, were that the oral exam wouldn't change his judgement. Orals can be subjective in any case, and he  perhaps wanted to avoid that as well as lessen his own work load in the face of it not very likely to affect the outcome. 

This would have been different if the suggested grade were lower than your expectations or if someone else were disadvantaged by the system. 

In my own practice, I would set a grading rubric that was clear to the students. For example, 700 points earned would be worth a certain grade. Students knew how many points they had accumulated. But if someone earned 695 points they would get the higher rather than the lower grade. My assumption was (a) that the difference was not consequential and (b) minor changes in evaluations along the way might overwhelm the few points lost. Furthermore, I would occasionally let students who had already attained highest marks skip the final exam. It would give them the opportunity to work in their other subjects and was unlikely to affect anything. I think this is somewhat common practice. 

Far worse to be overly strict and leave students disappointed in the course, in me, and in themselves. 

Furthermore, if the grade "offered" was less than highest marks, you had the choice to take the exam. No harm, no foul. 

I'm not sure I'd feel the same about this if the offer was made to only the lowest performing students. In that case it seems more like work avoidance than justice. But even then, I'd want more information before deciding. 

And congratulations. I hope that you impressed the professor in some way.