Q: My parish is planning for the initiation of our catechumens and candidates on a Sunday in Ordinary Time. Are we free to choose readings from among those in the Lectionary for Ritual Masses: Christian Initiation Apart From the Easter Vigil or are we bound to use those readings assigned to that particular Sunday of Ordinary Time?
A: Yes, you are so free.
RCIA 208 gives you permission to use the Ritual Mass for Christian Initiation – both the lectionary texts and the presidential prayers from the missal.
You’ll find an explanatory paragraph about the Ritual Masses in the Roman Missal just before the first one. It lists the days when ritual masses are prohibited. Sundays in Ordinary Time are not among them, meaning it’s fine to replace one of them with a ritual mass.
This is the same argument behind celebrating a wedding mass on a Saturday night in Ordinary Time.