The Trinitarian requirements of the Nicene creed , the form that this site takes as authoritative for "orthodoxy", doesn't specifically mention that issue though.
This is the creed of first council:
We
believe in one God, the Father Almighty, Maker of all things visible and invisible.
And in one
Lord Jesus Christ, the
Son of God, begotten of the Father [the
only-begotten; that is, of the essence of the Father, God of God], Light of Light, very God of very God, begotten, not made, being of
one substance with the Father;
By whom all things were made [both in heaven and on earth];
Who for us men, and for our
salvation, came down and was incarnate and was made man;
He suffered, and the third day he rose again, ascended into heaven;
From thence he shall come to judge the quick and the dead.
[But those who say: 'There was a time when he was not;' and 'He was not before he was made;' and 'He was made out of nothing,' or 'He is of another substance' or 'essence,' or 'The Son of God is created,' or 'changeable,' or 'alterable'—they are condemned by the holy catholic and apostolic Church.]
It was later expanded at the next council and they may be including this as part of the Nicene Creed as well but it still doesn't add anything an OT couldn't agree to.:
We believe in one God,
the Father, the Almighty,
maker of heaven and earth,
of all that is, seen and unseen. We believe in one Lord, Jesus Christ,
the only Son of God,
eternally begotten of the Father,
God from God, light from light,
true God from true God,
begotten, not made,
of one Being with the Father;
through him all things were made.
For us and for our salvation
he came down from heaven,
was incarnate of the Holy Spirit and the Virgin Mary
and became truly human.
For our sake he was crucified under Pontius Pilate;
he suffered death and was buried.
On the third day he rose again
in accordance with the Scriptures;
he ascended into heaven
and is seated at the right hand of the Father.
He will come again in glory to judge the living and the dead,
and his kingdom will have no end.
We believe in the Holy Spirit, the Lord, the giver of life,
who proceeds from the Father [and the Son],
who with the Father and the Son is worshiped and glorified,
who has spoken through the prophets.
We believe in one holy catholic and apostolic Church.
We acknowledge one baptism for the forgiveness of sins.
We look for the resurrection of the dead,
and the life of the world to come. Amen.
You might say that it would be illogical for an OT to adopt a belief in the Trinity. Maybe you believe that some of their philosphical beliefs about God should logically lead to a different conclusion? But if they don't actually follow those conclusions you think they should and still embrace the Nicene statement on the Trinity I don't see why they wouldn't be called Trinitarian.