Ordinary Time first readings have two purposes:
106. These readings have been chosen to correspond to the Gospel passages in order to avoid an excessive diversity between the readings of different Masses and above all to bring out the unity between the Old and the New Testament. The connection between the readings of the same Mass is shown by a precise choice of the headings prefixed to the individual readings.
To the degree possible, the readings were chosen in such a way that they would be short and easy to grasp. But care has been taken to ensure that many Old Testament texts of major significance would be read on Sundays. Such readings are distributed not according to a logical order but on the basis of what the Gospel reading requires. Still, the treasury of the word of God will be opened up in such a way that nearly all the principal pages of the Old Testament will become familiar to those taking part in the Mass on Sundays.
Readings from the New Testament have a different scheme:
107. There is a semicontinuous reading of the Letters of Paul and James (the Letters of Peter and John being read during the Easter and Christmas seasons).
Because it is quite long and deals with such diverse issues, the First Letter to the Corinthians has been spread over the three years of the cycle at the beginning of Ordinary Time. It also was thought best to divide the Letter to the Hebrews into two parts; the first part is read in Year B and the second in Year C.
Only readings that are short and readily grasped by the people have been chosen.
And this table may be found at the link.
The difficulty with aiming to lay (or clergy) understanding is that it seems easy to miss by being too simple or too deep. I think the Sunday Lectionary provides a decent balance. Perhaps what is needed even more today is a guide for preachers, a list of important notions covered in these readings. Less, I hope, offering preaching suggestions.
What are your observations about Ordinary Time scriptures on Sunday? I know there are issues with the selections. I’m not convinced that semi-continuous Paul or James or Hebrews is the best way to go. Sometimes I think the semicontinuous approach was just running close to deadline and that the Old Testament took more time and energy than first imagined.