There are set readings for each day. Unless your Priest was breaking the rules, then he had to of read from the old testament.
I remember the set readings. They would usually say from an epistle according to St Peter, or from the gospel.
I don’t really remember them reading the OT, although as I said, I was very young when I attended the Catholic church. About 10. And I was an atheist! so I didn’t pay that much attention.
What is the percentage, in your opinion, of OT readings during the year? I really am interested, because this is not the first time this has come up between me and other protestants.