What are the new improvements of OJ?

The new OJ saves all submissions so you don't have to worry about saving your code locally. The UI is redesigned from scratch to provide a much better user experience. There are many, many more improvements to come, stay tuned!

I have already signed up at leetcode.com, why can't I sign in to OJ?

You would need to create a new account at oj.leetcode.com. There is no plan to support account integration across these two sites.

I need to use the old OJ. Where can I find it?

Sorry, the old OJ had been retired and replaced by the new OJ.