Fr. John Hardon SJ is quoted at The Fatima Center defining these two sacramental terms:   Ex opere operantis is a term mainly applied to the good dispositions with which a sacrament is received, to distinguish it from the ex opere operato which is the built-in efficacy of a sacrament properly conferred.  

The above is an excellent definition of those two sacramental terms that always must be considered in a balance.  Most amateur theologians today are pretty good at knowing the principle of ex opere operato, namely, that a priest in mortal sin still validly confects the sacraments (provided he say the correct words.)  This is good that amateur theologians know this.  But most amateur theologians today are very ignorant of the balance struck between ex opere operato (literally, from the work performed) and ex opere operantis (literally, from the work of the doer) as elucidated by the Holy Fathers and Doctors of the Catholic Church.  Most modernists consider only the validity of the sacraments while ignoring the efficacy of the sacraments when done by, say, a holy priest versus an unholy priest.  Similarly, most modernists are blind to ancient considerations on the dispositions of the lay recipient in the sacraments.  We’ll consider two sacraments:  1) The Holy Sacrifice of the Mass and 2) Extreme Unction.

  1. The Holy Sacrifice of the Mass.  St. Thomas Aquinas teaches that the holy priest is “more readily heard” at Holy Mass than an unholy priest.  This is due to the theological principle of merit, a term that is greatly ignored in the highly Protestant culture that we live in.  Consider how Padre Pio got what he asked for at Mass since the continual prayer of a just man availeth much.—St. James 5:16 whereas even average Jews in Jesus’ day knew that God doth not hear sinners.—John 9:31.  Again, we’re evaluating the efficacy of sacraments here, not the validity of such sacraments. Of course, it would be the heresy of donatism to assert that a priest in mortal sin does not confecting valid sacraments.  Of course he does, provided he is saying the correct words.  Donatism is denying the ex opere operato aspect of sacraments.  But we also must consider the ex opere operantis aspects of the sacraments that, namely, a holy priest at Mass is “more readily heard” than an unholy priest at Mass.
  2. Extreme Unction.  The Carmelite book of spiritual par excellence written just before Vatican II is called Divine Intimacy with meditations all through the year.  One of the meditations recently was on Extreme Unction.  Fr. Gabriel OCD writes: A dying person who receives this Sacrament with the proper dispositions obtains the full remission of all his sins and of the penalty due to them, so that he can go directly from this earthly exile to eternal glory without passing through Purgatory.  But although this is the normal effect of Extreme Unction, very few actually receive all its fruit because very few have the proper dispositions.  There is perhaps no Sacrament which is so little understood.  It is often received in haste, without preparation, and in a state of complete or partial unconsciousness.  The result is that its precious fruits are lost in great part.—Divine Intimacy #74 (p. 216.)

Many amateur Catholics today know that the Apostolic Pardon eradicates Purgatory time if received on their death bed.  But they are almost all ignorant of the fact that the Apostolic Pardon only works if the person receiving it has the right preparations and dispositions.  As Fr. Gabriel in Divine Intimacy wrote above:  But although this is the normal effect of Extreme Unction, very few actually receive all its fruit because very few have the proper dispositions.  Thus, it is not donatism to recognize the ex opere operantis (dispositional) aspects of the holy sacraments, even as we readily admit the ex opere operato (automatic) aspect of the sacraments.  In short, all the saints would tell you that you need to go deep in prayer and penance and also go wide in faith and hope and charity if you want any of the sacraments to take any real effect in your life.  This is because the sacraments are not magic tricks.  The sacraments are grace building on nature.  Merit comes into this, and that is not pelagianism or donatism.  It’s the Magisterium of the Catholic Church.