From what i know OPro is a bit slimmer and harder to hit.

Opro II[1942]

fast, can go 44+ (with full gun) as long with good high level engies

got strong hitting guns

spread is near tight

slim hitbox

can be armorwhoring but not recommended...

got a bb5 sailor slots

Bismarck

can go 42  knts

got more range than Opro II

less damage than Opro

good to start learning on how to play KM style till

spread = fair

below the range of L1

You' re KM so you want range, bismarck ranges Lion 1 :)

The damage difference is from the 8 vs 9 barrels.

Bismark also have the h39 and h44' s guns setup and it's a beautiful ship. 

I like bismarck :D

Go for Opro 2. My setup is bissy guns coz as I have observed bissy guns damages more than the Opro 2 + it has more range and im running @ 45 knots with that setup. I feel like a Gneisenau with more range.

If you have access to the O Pro II 1942 use both. Why limit yourself? Anyone at level can use the EBB O Pro II. They are two different play styles so it can add some variety to your grind. I have both and I like them both. Start playing with one and depending on the type of players that are playing you can switch to the other if you need to.

OproII 1942 is an EBB, which means it is an BB3.5 which out tiers the Bismark at a solid BB3.  I have played both the OproII (non EBB) and Bismark and I must say the OproII is far superior for GB combat.  The little bit of extra range the Bismark has doesn't mean anything when you are still outranged by every BB45 in a GB.  OproII is slim, fast, and able to rush which makes it a fantastic ship in GBs. 

OproII 1942 is just an all around upgrade from the OproII with a few more knots and B5 supports.  

So OproII 1942 > OproII >> Bismark (In GBs)

Still keep both and use them for a fun grind.

The argument can be made that bismark prepares you for H39.  But still BB45 gameplay is a lot different from BB123 gameplay anyway.