The Old Testament never mentions God is one in the context of solitary oneness.
Yes there is...
Solitary.
Deuteronomy 32:39
New International Version
"See now that I myself am he!
There is no god besides me. I put to death and I bring to life, I have wounded and I will heal, and no one can deliver out of my hand.
New Living Translation
Look now; I myself am he!
There is no other god but me! I am the one who kills and gives life; I am the one who wounds and heals; no one can be rescued from my powerful hand!
English Standard Version
“‘See now that I, even I, am he, and
there is no god beside me; I kill and I make alive; I wound and I heal; and there is none that can deliver out of my hand.
Berean Study Bible
See now that I am He;
there is no God besides Me. I bring death and I give life; I wound and I heal, and there is no one who can deliver from My hand.
King James Bible
See now that I, even I, am he, and
there is no god with me: I kill, and I make alive; I wound, and I heal: neither is there any that can deliver out of my hand.
New King James Version
‘Now see that I, even I, am He, And
there is no God besides Me; I kill and I make alive; I wound and I heal; Nor is there any who can deliver from My hand.
New American Standard Bible
‘See now that I, I am He, And
there is no god besides Me; It is I who put to death and give life. I have wounded and it is I who heal, And there is no one who can save anyone from My hand.
NASB 1995
'See now that I, I am He, And
there is no god besides Me; It is I who put to death and give life. I have wounded and it is I who heal, And there is no one who can deliver from My hand.
NASB 1977
‘See now that I, I am He, And
there is no god besides Me; It is I who put to death and give life. I have wounded, and it is I who heal; And there is no one who can deliver from My hand.
Amplified Bible
‘See now that I, I am He, And
there is no god besides Me; It is I who put to death and I who give life. I have wounded and it is I who heal, And there is no one who can deliver from [the power of] My hand.
Christian Standard Bible
See now that I alone am he;
there is no God but me. I bring death and I give life; I wound and I heal. No one can rescue anyone from my power.
Holman Christian Standard Bible
See now that I alone am He;
there is no God but Me. I bring death and I give life; I wound and I heal. No one can rescue anyone from My hand.
American Standard Version
See now that I, even I, am he, And
there is no god with me: I kill, and I make alive; I wound, and I heal; And there is none that can deliver out of my hand.
Brenton Septuagint Translation
Behold, behold that I am he, and
there is no god beside me: I kill, and I will make to live: I will smite, and I will heal; and there is none who shall deliver out of my hands.
Contemporary English Version
Don't you understand?
I am the only God; there are no others. I am the one who takes life and gives it again. I punished you with suffering. But now I will heal you, and nothing can stop me!
Douay-Rheims Bible
See ye that I alone am, and there is no other God besides me: I will kill and I will make to live: I will strike, and I will heal, and there is none that can deliver out of my hand.
English Revised Version
See now that I, even I, am he, And
there is no god with me: I kill, and I make alive; I have wounded, and I heal: And there is none that can deliver out of my hand.
Good News Translation
"
'I, and I alone, am God; no other god is real. I kill and I give life, I wound and I heal, and no one can oppose what I do.
GOD'S WORD® Translation
See, I am the only God. There are no others. I kill, and I make alive. I wound, and I heal, and no one can rescue you from my power.
International Standard Version
"Look now! I AM, and
there is no other god besides me. I myself cause death and I sustain life; I wound severely and I also heal; from my power no one can deliver.
JPS Tanakh 1917
See now that I, even I, am He, And
there is no god with Me; I kill, and I make alive; I have wounded, and I heal; And there is none that can deliver out of My hand.
Literal Standard Version
See, now, that I [am] He, ""
And there is no god besides Me; I put to death and I keep alive; I have struck and I heal; And there is not a deliverer from My hand,
NET Bible
"See now that I, indeed I, am he!" says the LORD, "and
there is no other god besides me. I kill and give life, I smash and I heal, and none can resist my power.
New Heart English Bible
"See now that I, even I, am he, and
there is no god besides me. I kill, and I make alive. I wound, and I heal; and there is no one who can deliver out of my hand.
World English Bible
"See now that I, even I, am he,
There is no god with me. I kill, and I make alive. I wound, and I heal. There is no one who can deliver out of my hand.
Young's Literal Translation
See ye, now, that I -- I am He, And
there is no god with Me: I put to death, and I keep alive; I have smitten, and I heal; And there is not from My hand a deliverer,