Jesus felt that Isaiah was authoritative:
“But he answered and said, It is written, Man shall not live by bread
alone, but by every word that proceedeth out of the mouth of God.”
“Jesus said unto him, It is written again, Thou shalt not tempt the
Lord thy God.” (Matthew 4:7)
“Then saith Jesus unto him, Get thee hence, Satan: for it is written,
Thou shalt worship the Lord thy God, and him only shalt thou serve.”
We also have him quoting from Deuteronomy, compared to Matt 4:4 above, so he felt that it was authoritative.
Deuteronomy 8:3 (ESV)
And he humbled you and let you hunger and fed
you with manna, which you did not know, nor did your fathers know,
that he might make you know that man does not live by bread alone, but
man lives by every word that comes from the mouth of the Lord.
We also have the Jesus seemed to believe that Isaiah was written by only one author:
“that it might be fulfilled which was spoken by Esaias the prophet,
saying Himself took our infirmities, and bare our sicknesses.”
which related to
Isaiah 53:4 (ESV)
4 Surely he has borne our griefs and carried our sorrows; yet we esteemed him
stricken, smitten by God, and afflicted.
and then compare Matthew 1:22-23 with Isaiah 7:14, about the prophecy regarding the birth of Christ.
In Matt 12:1-8 we see Jesus affirming the truthfulness and reliability of 1Samuel 2:16, Exodus 25:30, Leviticus 24:5-9, Numbers 28:9-10, Hosea 6:6, Genesis 2:3.
Then we have Matt 19:1-6 where Jesus reaffirms the Genesis creation account, about God creating man and woman.
Then we have Matthew 22:41–46 where Jesus is stating that David was inspired
41 Now while the Pharisees were gathered together, Jesus asked them a question,
42 saying, “What do you think about the Christ? Whose son is he?” They said to him, “The son of David.”
43 He said to them, “How is it then that David, in the Spirit, calls him Lord, saying,
44 “ ‘The Lord said to my Lord,
“Sit at my right hand,
until I put your enemies under your feet” ’?
45 If then David calls him Lord, how is he his son?” 46 And no one was able to answer him a word, nor from that day did anyone dare to ask him any more questions.
and in Matt 27:35 where Jesus sees David as a prophet.
And they crucified him, and parted his garments, casting lots: that it
might be fulfilled which was spoken by the prophet, They parted my
garments among them, and upon my vesture did they cast lots.
So by going by just one Gospel we can see that Jesus felt the OT was authoritative, at least a representative sample of the OT.