While we can’t receive communion at an Orthodox service, can one fulfill the Sunday obligation to worship God by attendance since the Orthodox churches are recognized as true cchurches with valid orders and sacraments?
Generally speaking, no a Catholic does not fulfill his/her Sunday obligation by attending an Eastern Orthodox Church’s Divine Liturgy.
1248 §1. A person who assists at a Mass celebrated anywhere in a Catholic rite either on the feast day itself or in the evening of the preceding day satisfies the obligation of participating in the Mass.
Note that the canon specifies that it must be a Catholic rite in order to fulfill the obligation.