I don't suppose it's been open-sourced, has it? It would be great to have an alternative browser engine out there, instead of just three (or two and a half, depending on how you count Blink and Webkit).
No, it was leaked illegally. Opera filed a lot of DMCA complaints with many online Git repository services to get the code removed from them. But you can still find it if you Google hard. I also recall reading that some Russian developers were working on the leaked code and had also released a "newer" version with some patches (Opera presto was really popular in Russia) but I couldn't find it.
There is another little known open source browser project independent of Gecko, webkit and Blink currently being developed but I just can't remember its name, even though I tried compiling it on my system ... damn ... I'll post an update when its name comes to mind.