The textbook had -oi- in lots of places the text that the French book had -ei- in, i.e., veoir, manoit, estoit instead of veeir, maneit, esteit. There were other differences, and the textbook discussed the regional variations of Burgundian, Picard, Anglo-Norman and other dialects of Old French and the general rules for changes.

I also noted that Marie de France used the word gupil for "fox", as the Reinardus Vulpes stories hadn't yet eclipsed the Old French word.

I was going to ask if "fiz a" (or, rather, fils a) was something that was acceptable in modern French, so I'm glad you mentioned that.

I'm certainly going to read more Old French. I was thinking trying one of the Arthurian stories.