The Ecumenical Patriarchate has a long standing policy of allowing Old Calendar parishes and jurisdictions under it, to follow either calendar they chose, considering the calendar not an issue. Mount Athos which is under its jurisdiction, being an example of that policy; the Ukrainian eparchies in the West, and ACROD are other examples. The St. Irene Chrysovalontou Monastery in Astoria, Long Island, was a noted bastion of Old Calendarism when it was accepted by the Ecumenical Patriarchate in 1997. (It is currently under investigation by the patriarchate and civil authorities for scandalous allegations, the Abbot has resigned, etc.)
Remember, however, many Old Calendarists consider the calendar only one reason for their separation, so called "modernism" and ecumenical activities being among the primary reasons.