I have a couple of questions to ask.

Question 1 : When you load from a save when you get this mod?

Question 2 : If yes, Do you recommend starting a new game? Or just load from the save?

Question 3 : Is this mod worth getting? Do you recommended it? Did you enjoy it?

1: You can start using this mod whenever, but you will need to either run the Update Savegame program that comes with OOO or use Wrye Bash to update NPC levels, then go to a safe cell (use coc testinghall in the console) and wait for 4 days so the cells respawn (or whatever time you have set for the cell reset).

2: See above, but it is recommended to start a new game if this will be your first time playing OOO.

3: Yes.

http://devnull.sweetdanger.net/OOO/OOO_Guide.html#upSave and updating NPC levels with Wrye Bash are two different things. Running OOOupsave is only needed if going from a save made with a pre-OOO 1.32 game to OOO 1.32+ game, i.e. from an OOO.esp to an OOO.esm game. In short, seldom necessary as there should be few people running pre-1.32 versions of OOO and not if adding OOO to an ongoing game without it.

Updating NPC levels via Wrye Bash is necessary if adding OOO to an ongoing game, though. Otherwise NPC's will have vanilla stats rather than OOO stats.

I'd definitely recommend to start OOO with a new game.