Hi Trev,
I've had problems avoiding CTDs at beginning of missions, so I put OH II as the last mod, and now it's running stable.
Strangely though, when leaving Brest for The Black Pit (wolfpack mission) there is not a single ship in or outside the Brest harbour area, except for 2 Uboats in the neighbouring bunkers. The area is totally empty. FPS very high (of course, since nothing for the hardware to chew). It's the same with mines, ice & dolphins enabled or disabled - no difference.
Further ahead along the route to the mission patrol area there is some enemy traffic, and also the occasional fellow Uboat heading westward. I sank a British small merchant and continue westward. So OH II's mission traffic itself around the Atlantic seems OK, but life/traffic around the harbour area seems nuked, dead. During the mod testing phase this was not so.
I started a separate mission out of Brest with the same mod loadout but
without OH II, for comparison. Harbour traffic was there, as normal. But when I added OH II (new campaign) harbour traffic vanished.
Thought I'd mention this.