The Church of Genuine Orthodox Christians (GOC) of Greece stood for the truth in the 1920s when the Greek church at large took up the new calendar. The GOC did not, and suffered serious persecutions at the hands of the mainline Greek church for decades. You can read about it on the GOC – About Us and GOC – A Brief History pages from their website.
The GOC would have entirely died out in the 60s were it not for timely support and recognition from the Russian Orthodox Church Abroad (ROCA) and the Monastery of Esphigmenou on Mount Athos. The GOC and ROCA (also known as the Russian Orthodox Church Outside of Russia, or ROCOR) remained in communion for some time.
But in Greece the GOC, continuing under persecution until 1986, was not recognized legally as a church and had to hide its assets under various private or other legal entities. Finally in 1986 it became legally recognized, but it was another ten years before the newly re-constituted synod of twelve bishops attempted to consolidate its property ownership. This was accomplished in 1995, and Archbishop Chrysostomos II was elected to lead the Church. But the effort unfortunately led to the defection of six of them. (Of the defecting group, dubbed the Lamians, only two remained together and chose Bishop Makarios to be their leader as Archbishop of Athens. They united with HOCNA in 2013.)
In 2014 GOC absorbed another Greek synod known as the “Cyprianites” into itself, and entered into communion with the Romanian Old Calendar Orthodox Church, and ROCA under Agafangel (that portion of the ROCOR that did not re-enter communion with Moscow in 2007), and others manifesting opposition to the heresy of Ecumenism. (A Response to some Distortions by Archbishop Gregory Concerning the GOC)
Here are the webpages of ROCA.
Or visit the Agape Community outside of Liberty, TN