Here are some answers to questions about compatibility with various mods asked about on the forums (Skyrim SE and Skyrim Classic together)
... Alpine Forest of Whiterun Valley - yes (OCW does not change the Whiterun hold campaign)
... Bells of Skyrim - yes
... Boethiah's Bidding - yes (at least for OCW versions up to 2.6.2)
... Cities of the North - has patches, for example COTN Dawnstar has patch https://www.nexusmods.com/skyrimspecialedition/mods/30885?tab=files
... City Entrances Overhaul - Markarth - see details https://www.nexusmods.com/skyrimspecialedition/mods/11076/?tab=forum&topic_id=7296071
... Civil War Champions (from Creation Club) - no complaints
... Civil War Equipment organization - yes
... Arthmoor's Villages - yes
... Diverse Skyrim - yes
... "Experience" mod - yes
... "For the stormcloak's" - no, not really. Also, see https://www.nexusmods.com/skyrim/mods/90233
... The Great City series - abc123guy@ reported "for example main entrance to Dawnstar screws up the pathing of invaders. That's one instance I saw though, there could be more or just the one."
... Helgen Reborn - yes, and OCW may even update the shape of the flag from square "fort" to triangle "town"
... Holds the City Overhaul - in some minor holds, city walls and gates derail civil war battles in cities
... Immersive Citizens AI Overhaul (ICAIO) - yes
... Immersive Patrols - yes
... Inigo - yes - Inigo will try to go back to his starting cell during the battle of Riften
.... JK's Skyrim - yes
... Open Cities - yes it is compatilbe - it is expected that Open Cities will revert to closed form cities in major hold battles (Whiterun, Solitude, Windhelm) and with OCW also Markarth and Riften, and also defenses.
... Organized Bandits of Skyrim - yes
... Palaces And Castles Enhanced - yes it says so there - BEWARE that mod breaks vanilla civil war - Battle of Windhelm - randomly with a fair chance (because it adds more gates in Windhelm tagged as civil war gates and the quest CWSiege picks gates randomly, if it does not pick the correct main gate then the battle will be broken)
... Perkus Maximus - yes - maybe apply its patcher
... "Populated ..." mod series - yes, these are completely different kind of mods
... Radiant Spawn Engine - yes
... Requiem - yes - maybe apply Reqtifiqator
... The Second Great War - yes
... Skyrim Unbound - yes
... Warzones - compatible - they are pretty much independent in what they do.
Would compatibility with Fort Takeovers/Realistic Conquering be possible? Adding the various locations you can take over using them to the war map and everything?
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Is OCW compatible with ... ? (7 comments)
... Alpine Forest of Whiterun Valley - yes (OCW does not change the Whiterun hold campaign)
... Bells of Skyrim - yes
... Boethiah's Bidding - yes (at least for OCW versions up to 2.6.2)
... Cities of the North - has patches, for example COTN Dawnstar has patch https://www.nexusmods.com/skyrimspecialedition/mods/30885?tab=files
... City Entrances Overhaul - Markarth - see details
https://www.nexusmods.com/skyrimspecialedition/mods/11076/?tab=forum&topic_id=7296071
... Civil War Champions (from Creation Club) - no complaints
... Civil War Equipment organization - yes
... Arthmoor's Villages - yes
... Diverse Skyrim - yes
... "Experience" mod - yes
... "For the stormcloak's" - no, not really. Also, see https://www.nexusmods.com/skyrim/mods/90233
... The Great City series - abc123guy@ reported "for example main entrance to Dawnstar screws up the pathing of invaders. That's one instance I saw though, there could be more or just the one."
... Helgen Reborn - yes, and OCW may even update the shape of the flag from square "fort" to triangle "town"
... Holds the City Overhaul - in some minor holds, city walls and gates derail civil war battles in cities
... Immersive Citizens AI Overhaul (ICAIO) - yes
... Immersive Patrols - yes
... Inigo - yes - Inigo will try to go back to his starting cell during the battle of Riften
.... JK's Skyrim - yes
... Open Cities - yes it is compatilbe - it is expected that Open Cities will revert to closed form cities in major hold battles (Whiterun, Solitude, Windhelm) and with OCW also Markarth and Riften, and also defenses.
... Organized Bandits of Skyrim - yes
... Palaces And Castles Enhanced - yes it says so there
- BEWARE that mod breaks vanilla civil war - Battle of Windhelm - randomly with a fair chance (because it adds more gates in Windhelm tagged as civil war gates and the quest CWSiege picks gates randomly, if it does not pick the correct main gate then the battle will be broken)
... Perkus Maximus - yes - maybe apply its patcher
... "Populated ..." mod series - yes, these are completely different kind of mods
... Radiant Spawn Engine - yes
... Requiem - yes - maybe apply Reqtifiqator
... The Second Great War - yes
... Skyrim Unbound - yes
... Warzones - compatible - they are pretty much independent in what they do.