The server is still up, but it was dead the last few times I tried to play it sometime last year. DOA5LR being free on Steam now probably killed off what population it had left. You might need to directly hit up some people on here for matches if you want to play.
If you'd prefer to play straight up DOA2U as it was on Xbox, without the character changes from DOAO, you can actually replace all of the character .bin files in DOAO with the ones from DOA2U. As long as you and your opponent are both using the 2U files, it'll work. So no more blockable Dragon Kicks.