“And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me. Then opened he their understanding, that they might understand the scriptures.” (Luke 24:44-45)
The Law of Moses, prophets, and psalms comprise the whole Old Testament.
Everything said about Jesus must happen.
Many of these things have already happened.
There are still things to happen.
They will happen.
We must understand the Old Testament in its plain, normal sense if we are to KNOW if something is fulfilled. If we interpret the Bible in some other sense as allegorical, then we will not know if something has happened or not.