Generally when people say old testament or covenant, they are referring to the ten commandments only.
When someone says the old testament is obsolete, it causes confusion. Because one will ask, so are you saying that we can now kill or covet our neighbours posessions?
As we read through the new testament, we see the same commands as the ones listed in the ten commandment so how is it obsolete?
The law is obsolete in the sense that we now have a new version of the same old commandments. If someone has a written contract with someone which should last for a certain amount of time, when the period has ended, a new one has to be written even if it has the same info as the previous one. If changes are to be made to the contract before it is fulfilled, one cant just cross out words or add words. A new contract has to be written with the updated info. Same thing with God's command.
So God fulfilled the law or contract that He had with Israel, thus the contract came to an end. And using that same old contract as a foundation He instituted a new covenant. So yes, the old and the new are almost the same, but with amendment.
We are not comnanded to observe sabbath day, but we see that when we trust in God we enter the rest of God.
Jesus died for our sins, and all we need for salvation is to trust in Him, yet still He says dont use your freedom as excuse for badness.
In Old testament it says, dont be overly righteous or overly evil. The same sentiments are expressed in new testament when He says be perfect, as your Father in heaven is perfect, but He also says if anyone says he is without sin he is deceived.
So its less confusing not to think of the law as being obsolete, but to say it is amended. The amended law being the new one.
Paul says we must use the israelites as an example of what happens when we are disobedient. 1Corinthians 10:11 Now all these things happened to them as examples, and they were written for our admonition, upon whom the ends of the ages have come.
God does not change, therefore the OT and the NT are similar, and some aspects of the OT can be used as inspiration in helping us to be obedient to the new covenant.
Jesus says to forgive, and when we read the story of Joseph and His brothers we see a powerful story of forgiveness.
Under the old covenant, if they did not obey God, they would get a severe punishment. But at this time it is a time of grace. Never the less, we are to obey God. But in doing the will of God, it is not to gain salvation, but we do the will of God because we are His children and we want to please Him.