Isn't it obvious that the apostles taught from the OT? How many times do they quote the OT in the New? The Septuigent and the Hebrew scriptures were both available.
They taught what they had learned from Christ
Which was the OT! Luke 24:27 - "Then beginning with Moses and with all the prophets, He explained to them the things concerning Himself in all the Scriptures."
The Bible as we know it today wasn't formalized until 400 AD. What did they use?
The canon was established well before 400 AD. What did the apostles quote from? The entire NT was written before 100 AD. How could they quote from the OT if it didn't exist until 400? What did Jesus quote from? What did He read in the temple? The Jerusalem Times?
The OT Scriptures were referenced a lot but not so much for their message or the law, but for their 450+ references to and prophesies of Jesus and His ministry and primarily in the context of evangelizing Jewish communities. They were rarely referenced with respect to the New Law and had little traction with the Gentile communities.
As for their being available and recognized by the late first century is really two different issues. There were many competing writings and forgeries so the challenge was to identify the authentic writings from among the many others. This is not unlike you having to pick the correct six lottery numbers from the numbers from a simple list of numbers 0 through 50.