I can do that in my Linux system.
1. Install the program fontforge.
2. Create a script called e.g. otf2ttf.sh in /home/$USER/bin, with the following content:
#!/usr/bin/fontforge
i=1
while ( i<$argc )
Open($argv[i])
Generate($argv[i]:r + ".ttf")
i = i+1
endloop
3. Make this script executable.
4. Open a terminal in the directory with your .otf file(s) and enter the following command:
for i in *.otf; do echo $i; fontforge -script ~/bin/otf2ttf.sh $i; done
5. Profit!
[Edit]
Fontforge is available for Windows as well, but I have no idea how to proceed there...