This topic is just sharing.
I just setup OCS on my ubuntu server. By default the system does not enable captcha on user registration process. Enable this captcha should prevent auto registration by spam/bootware.
Step by step :
sudo apt-get install php5-gd fontconfig
sudo nano ~./ocs/config.inc.php
font_location = /usr/share/fonts/truetype/freefont/FreeSerif.ttf
font_location = /usr/share/fonts/truetype/dejavu/DejaVuSerif-Bold.ttf
sudo service apache2 restart
if you want to use FreeSerif.ttf of another font, you can download from here then send over file transfer to your server.