While in theory, any Eastern Orthodox church is in communion with any other, the reality is that, at any given point, at least one of them refuses to commune members of another one of them…
In theory, yes, any EO who is properly prepared can be communed at any EO church parish. In practice? Not really.
The EO officially consider anyone not in an EO church to not be eligible for any sacraments, period.
The Oriental Orthodox Communion (those under the Pope of Alexandria) are similar; everything I’ve seen says they are less rigid about it than the EO, but the OO are still more rigid than the Catholics as a rule.
The Assyrian Church of the East (ACE), Armenian Apostolic Orthodox and the Syrian Orthodox officially have limited intercommunion with the Catholic Church via their parallel catholic churches (Chaldean, Armenian, and Syrian).
The various Ukrainian Orthodox Churches are in an odd situation. The KP are not in communion with the EO officially, the MP are, and the UAOC are not in communion with either, but will commune members of both…