The following scriptures may help to clear up your misunderstanding about John the Baptist being part of the OT: "The law and the prophets were until John: since that time the kingdom of God is preached, and every man presseth into it." (Luke 16:16)
"The law and the prophets were until John - The law and the prophets continued to be the sole teachers until John came, who first began to proclaim the glad tidings of the kingdom of God:" Adam Clarke's Commentary
Mark 1:1-5 clarifies that John the Baptist was a NT prophet who began preaching the gospel of Jesus Christ: "The beginning of the gospel of Jesus Christ, the Son of God; As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee.
The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins.
And there went out unto him all the land of Judaea, and they of Jerusalem, and were all baptized of him in the river of Jordan, confessing their sins.
As referenced in Luke 24:47, Jesus said that repentance and remission of sin would be preached in His name in all nations beginning in Jerusalem. This happened on the Day of Pentecost when Peter said everyone must repent, and be baptized in the name of the Lord Jesus for the remission sin, (water) and you shall receive the gift of the Holy Ghost. This is when water baptism was begun in the name of the Lord Jesus in association with His death, burial and resurrection.
Confirmation that being baptized in the name of the Lord Jesus is water baptism is seen in (Acts 2:38; 8:12-18, 10:47-48)
If you notice in Acts 19: 1-6, it was after Paul baptized the group in the name of the Lord Jesus (water) that he then laid hands on them in order for them to receive the Holy Ghost.