If we spend just a few minutes and install FontForge and make a very small script it becomes almost trivial to do.
sudo apt-get install fontforge
Now we should make the scripts, well it is two actually but one is a small config script for FontForge and the other is just a script to convert many OTF files in one go, rather than individually. First the FontForge settings file.
Then open otf2ttf.conf in your favourite text editor and paste the following into it.
# Quick and dirty hack: converts a font to truetype (.ttf)
Save the file, and now lets make the script to convert many fonts in one swoop.
chmod +x otf2ttf
Again, open this file with your prefered text editor and paste the following in.
# had to enable extglob
shopt -s extglob
# this uses globbing to match fiels ending in otf/OTF
for i in +(*.otf|*.OTF)
do fontforge -script /home/subbass/bin/otf2ttf.conf $i
To use this now (provided that your ~/bin folder is in your path) just open a terminal where your OTF files are, and issue the command otf2ttf. It will convert 20 files in just a couple of seconds to give you an idea of speed, you can then move all the resulting TTF files into ~/.fonts
Credit to http://www.stuermer.ch/blog/convert-otf-to-ttf-font-on-ubuntu.html where I found the script before re-typing the instructions. I put it here so I don't lose it next time I need it.