greek syntax principle 21

A relative pronoun agrees with its antecedent in gender and number. Its case is determined by its function in the clause it begins (cf. BBG 14.10).

Matthew 1:23 Matthew 1:23
Ἰδοὺ ἡ παρθένος ἐν γαστρὶ ἕξει καὶ τέξεται υἱόν, καὶ καλέσουσιν τὸ ὄνομα αὐτοῦ Ἐμμανουήλ, ἐστιν μεθερμηνευόμενον Μεθʼ ἡμῶν ὁ θεός. “Behold, the virgin ⌊will become pregnant⌋ and will give birth to a son, and they will call his name Emmanuel,”which is translated, “God with us.”

Notice first that ὅ is masculine, singular which agrees with its antecedent Ἐμμανουήλ which is also masculine, singular.
Then notice that ὅ is also the subject of the verb ἐστιν and hence is in the nominative case.