The original question related to which Churches were 'Old Calendar"?
This term I confess irritates me somewhat. The Julian calendar is the calendar of the Orthodox Church, it is the Church Calendar. There is a novel and new 'calendar' introduced 'uncanonically' in 1924. This arbitary action, undertaken for all the wrong reasons, rent the liturgical unity which had been achieved over centuries by many Synods, including at least one here in England.
The Russian, Serbian, Jerusalem and Sinai Churches continue to use this the Church calendar.
So there is a Church calendar and a new or 'Revised Julian Calendar', and in Finland the Gregorian calendar.
Who adheres faithfully to that precious pearl of Faith that we are supposed to receive and pass on with nothing added and nothing taken away is a question which is far less easy to answer. Is ROCOR about to do about turn and deny its' witness of 84 years, who knows?
These are difficult times. I have no simple answer to offer, sadly.