For anybody struggling to install this mod, here's a guide:
Make sure you have Half-Life and Opposing Force installed. Download unblkmoon.zip from Moddb. Extract the contents of the .zip file to your Opposing Force "maps" folder.
In my case, it is "C:\Program Files (x86)\Steam\steamapps\common\Half-Life\gearbox\maps".
Then run Opposing Force, open the console, and type "map force300" (no quotes) to start the mod.
agreenman
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