The binary from oolite.org indeed works well, thanks!
I tried to naively replace /usr/share/oolite/oolite (from AUR package) with /home/crem/GNUstep/Applications/Oolite/oolite.app/oolite (installed from the binary package from oolite.org), but it wasn't able to find libgnustep-base.so.1.20 then (I have 1.27 installed system-wide instead).