Crash on game load.
Muluna triggers this specific crash, other mods (a lot) fine with it.
As far as AI knows, result_amount was removed in 2.0 (referencing https://www.reddit.com/r/factorio/comments/1ubvsg5/how_to_modify_smelting_results_mod_writing_help/)

Both mods work fine standalone.

Failed to load mods: OCs_base_assets/prototypes/utils/recipe-generator.lua:460: result_amount is invalid: nil for recipe casting-steel
stack traceback:
[C]: in function 'error'
OCs_base_assets/prototypes/utils/recipe-generator.lua:460: in function 'resolve_materials'
OCs_base_assets/prototypes/utils/recipe-generator.lua:627: in function 'single_recipe_generator'
OCs_base_assets/prototypes/utils/recipe-generator.lua:774: in function 'batch_generator'
...ing__/prototypes/compatibility/aai-vehicles-ironclad.lua:16: in main chunk
[C]: in function 'require'
OCs_ammo_casting/data-updates.lua:17: in main chunk

result_amount is just an intermediate variable in my code, and it is an error by design. It tells me the other mod isn't fully compatible yet. I will investigate the Muluna compatibility on my side this week. I can offer a hotfix sooner, though.