Do Orthodox believe in the Assumption of the Blessed Virgin which is a Roman Catholic dogma?
Orthodox Christians observe the feast of the Dormition or Falling Asleep of the Theotokos on the same day (adjusted to calendar if applicable) as the Roman Catholic Church, and some Orthodox parishes are named “Assumption of the Virgin / Theotokos”) although this is more of a Western expression. Orthodox Christians believe, not as a dogma but as an ecclesiastical conviction that the Theotokos was not in any sense of form captive to death and thus translated to the fullnest of life in Christ, body, soul and spirit. This belief in not dogmatized in the Orthodox Churches (as it was done by the Roman Catholic Church in 1950).
A recommended resource is: Ancient Traditions of the Virgin Mary’s Dormition and Assumption by Stephen J. Shoemaker.