Exodus 19:3
Then Moses went up to God, and the LORD called to him from the mountain...
Ever wonder why the word God and the LORD are used, seemingly interchangeably? Well, we must go back to the Hebrew words to understand. The word God is Eloheim (meaning Father, Son, and Holy Spirit).  The word LORD is Yehovah (meaning Jesus). Moses went up the mountain to Father, Son, and Holy Spirit but is was the Son who spoke to him. Using the meanings of the Hebrew words gives us a better vision. This same thing is especially true with the following verse:
Exodus 19:16
On the morning of the third day there was thunder and lightning, with a thick cloud over the mountain and a very loud trumpet blast.

Now if we look only at the English words, we envision a storm on the mountain, black heavy clouds, booming thunder, and flashes of lightning. But using the Hebrew word meanings, we get a completely different picture. Here's how it would read in the Hebrew.

"There were loud voices in proclamation blowing a great trumpet accompanied by bright, glittering gleams enveloped in bright clouds."

Does that sound a bit like the company of angels that announced the birth of Jesus to the shepherds??? It should. Just read these verses: Dan 7:9-10, Matt 17:2-5, Acts 7:53, Gal 3:19, Heb 2:2, Rev 4:1, Rev 8:13, Rev 9:14.

God spoke the words of the Ten Commandments the first time. The people did not want to listen to the voice of God.  They didn't want to get too close to God.  A similar thing happened with Jesus.
John 12:28-29
Father, glorify your name!" Then a voice came from heaven, "I have glorified it, and will glorify it again."The crowd that was there and heard it said it had thundered; others said an angel had spoken to him.

Who is this "angel" God sent to go with them who has the power to forgive sin?
1 Cor 10:1-4
For I do not want you to be ignorant of the fact, brothers, that our forefathers were all under the cloud and that they all passed through the sea.They were all baptized into Moses in the cloud and in the sea. They all ate the same spiritual food and drank the same spiritual drink; for they drank from the spiritual rock that accompanied them, and that rock was Christ.
John 5:22
Moreover, the Father judges no one, but has entrusted all judgment to the Son

Pretty clear...the "angel", God's Presence, was God, the Son.

Another example of how God, the Son interacted with people in the Old Testament is found in this passage.
Exod 24:9-11
Moses and Aaron, Nadab and Abihu, and the seventy elders of Israel went up and saw the God of Israel. Under his feet was something like a pavement made of sapphire, clear as the sky itself. But God did not raise his hand against these leaders of the Israelites; they saw God, and they ate and drank.

And then we have this verse.
John 6:46
No one has seen the Father except the one who is from God; only he has seen the Father.

If no one can see the Father then how can Moses, Aaron, Nadab, and Abihu, along with the seventy elders see God? Again, it's perfectly clear... the leaders saw God, the Son.