When used with your foundry expansion mod the game crashes during startup with the following error:
Failed to load mods: File OCs_base_assets/graphics/technology/overlayer-tech-lava-droplet.png not found

I redownloaded the latest version for 2.1 using the in game mod manager and opened up the archive to confirm the image is still missing in the latest version

When used with your foundry expansion mod the game crashes during startup with the following error:
Failed to load mods: File __OCs_base_assets__/graphics/technology/overlayer-tech-lava-droplet.png not found

I redownloaded the latest version for 2.1 using the in game mod manager and opened up the archive to confirm the image is still missing in the latest version

Thanks for the bug report. Which technology is it exactly? The log should tell us.
Also, do you have the latest versions of the other OC mods, like stone casting?

Sorry for duplicating my previous message, must've misclicked without noticing. I have the latest version of all the mods, and found out when trying to get a base case to recreate the issue that the startup error is gone. Looking at the logs and specifically this error

10.385 Error AtlasBuilder.cpp:1378: File __OCs_base_assets__/graphics/technology/overlayer-tech-lava-droplet.png not found; mods: OCs_foundary_expansion
         Prototypes using the sprite:
           data.raw["technology"]["factory-casting-1"]
           data.raw["technology"]["factory-casting-2"]
           data.raw["technology"]["factory-casting-3"]
           data.raw["technology"]["factorissimo-casting-space-factory"]

makes me think this is factorissimo throwing an error when trying to mess with icons that would appear on the factory buildings? I have no idea if this is something you want to look at for compatibility but I did confirm removing it from my modlist resolved the error.

Thanks for the details. This is part of my compatibility code between my "main" mod and factorissimo. I'll make a hotfix for that later this day.

Version 0.6.3 of OCs Exoplanetary Expansion should fix this issue.