Under Big Sur 11.5.1 the app crashes on launch with the following errors in the Console:

Aug 11 21:41:35 ** Divinity - Original Sin[52249]: getattrlist failed for /System/Library/Extensions/AppleIntelKBLGraphicsGLDriver.bundle/Contents/MacOS/AppleIntelKBLGraphicsGLDriver: #2: No such file or directory
Aug 11 21:41:35 ** Divinity - Original Sin[52249]: getattrlist failed for /System/Library/Frameworks/OpenGL.framework/Resources//GLRendererFloat.bundle/GLRendererFloat: #2: No such file or directory
Aug 11 21:41:36 ** Divinity - Original Sin[52249]: objc[52249]: Invalid or prematurely-freed autorelease pool 0x10280a090. Set a breakpoint on objc_autoreleasePoolInvalid to debug. Proceeding anyway because the app is old. Memory errors are likely.

The game can successfully be launched by:
- right-clicking the app and selecting "Show Package Contents".
- Navigate to "Contents/MacOS/"
- Double click "Divinity - Original Sin"

Just passing it along in case it's helpful for someone else. (I just made an alias of the above target file beside the regular app icon.)