before Vatican II priests used to say Mass alone a lot.
Can this be done using the OF Mass?
I mean can priests still say private masses alone in the OF?
Sometimes.
CIC (Latin Canon Law)
Can. 902 Unless the benefit of Christ’s faithful requires or suggests otherwise, priests may concelebrate the Eucharist; they are, however, fully entitled to celebrate the Eucharist individually, but not while a celebration is taking place in the same church or oratory.
Can. 903 A priest is to be permitted to celebrate the Eucharist, even if he is not known to the rector of the church, provided either that he presents commendatory letters, not more than a year old, from his own Ordinary or Superior, or that it can be prudently judged that he is not debarred from celebrating.
Can. 904 Remembering always that in the mystery of the eucharistic Sacrifice the work of redemption is continually being carried out, priests are to celebrate frequently. Indeed, daily celebration is earnestly recommended, because, even if it should not be possible to have the faithful present, it is an action of Christ and of the Church in which priests fulfil their principal role.
Can. 905
§1 Apart from those cases in which the law allows him to celebrate or concelebrate the Eucharist a number of times on the same day, a priest may not celebrate more than once a day.
§2 If there is a scarcity of priests, the local Ordinary may for a good reason allow priests to celebrate twice in one day or even, if pastoral need requires it, three times on Sundays or holydays of obligation.
Can. 906 A priest may not celebrate the eucharistic Sacrifice without the participation of at least one of the faithful, unless there is a good and reasonable cause for doing so.