TVHM and UVHM is pretty much ng+ and ng++ OP levels are different though, when you reach level 72(assuming you have all DLC for it), you can start going into OP levels. To unlock OP1, you go through Digistruct Peak, which is essentially all bosses in the game in on one huge mountain which you keep going up which gets more difficult the higher you get. OP1 means all enemies will be a level above you, but so will all gear. This keeps repeating until OP8, which is the last OP level. Essentially, OP levels is just everything but with more numbers, pretty much Gearbox's last minute solution for keeping the game running longer. You can go through all OP levels if you want but its not worth it IMO, unless you want to collect pearlescent weapons, one is guaranteed when you kill the last boss of the Peak. Pearles suck anyways.