This is from my article 'The Three Charisms of the Sacred Magisterium'
Joseph Cardinal Ratzinger, Prefect of the Congregation for the Doctrine of the Faith, stated, in a public letter dated October 28, 1995, that the teaching of Ordinatio Sacerdotalis is from the Sacred Deposit of Faith: “founded on the written Word of God, and constantly held and applied in the Tradition of the Church.”  He also states that this teaching is “explicitly addressed to the entire Catholic Church,” and that “all members of the faithful are required to give their assent to the teaching stated therein.”  Thus, the Prefect agrees that Ordinatio Sacerdotalis meets those two criteria (numbered 4 and 5 above) for a teaching under Papal Infallibility. The first criterion, that the infallible statement be issued only by the Roman Pontiff, is also obviously met.
However, in the same letter, Cardinal Ratzinger states his opinion that Ordinatio Sacerdotalis does not fall under Papal Infallibility: “In this case, an act of the ordinary Papal Magisterium, in itself not infallible, witnesses to the infallibility of the teaching of a doctrine already possessed by the Church.”  The “Responsum ad Dubium” to which the Prefect's letter refers, was approved and authorized by the Pope. But the Responsum only reaffirms that the teaching of Ordinatio Sacerdotalis belongs to the Sacred Deposit of Faith; it does not state explicitly whether or not the teaching falls under Papal Infallibility.
Interestingly, the Prefect, in his commentary after the “Responsum ad Dubium,” affirms the remaining two criteria for a teaching to fall under Papal Infallibility. Ratzinger refers to the ex cathedra requirement: “the Roman Pontiff, exercising his proper office of confirming the brethren (cf. Lk 22:32)….”  This wording mirrors that found in the Vatican II teaching on the ex cathedra requirement for Papal Infallibility, including citing the same Scripture passage.  Ratzinger also calls the teaching in Ordinatio Sacerdotalis, “a formal declaration, explicitly stating what is to be held always, everywhere, and by all….”  The Prefect's statement has the same basic meaning as the Vatican II wording: “by a definitive act, he proclaims….”  Thus, the third criterion for a statement under Papal Infallibility is also met.
Yet, in his letter following the “Responsum ad Dubium,” the Prefect maintains that the teaching does not fall under Papal Infallibility. He is clearly mistaken, since, in the Responsum and in the letter that accompanied it, his own words point out each of the criteria needed for a declaration under Papal Infallibility. His misunderstanding seems to come from the fact that the Pope was reaffirming a prior teaching of the Church. But all teachings of the Sacred Magisterium, including those given under the first charism of Papal Infallibility, are simply restatements or clarifications of prior teaching always present within the Sacred Deposit of Faith. The fact that a teaching has been previously taught by the Church does not prevent that teaching from being taught and reaffirmed under Papal Infallibility.
Because of this Responsum and the letter that accompanied it, some theologians hold that the teaching of OS does not fall under Papal infallibility. The problem with that position is that teachings under Papal Infallibility are infallible in and of themselves, by meeting all of the criteria taught by Vatican I, regardless of subsequent opinions or statements, even from the Pope or from the CDF.
A Pope does not need to intend to exercise Papal Infallibility, and he does not need to know that he is teahcing infallibly in order for the teaching to be infallible. All that is necessary is that each of the criteria be met.
Roman Catholic theologian