Apache mod_rewrite is required for Ossn to work.
If you receive error during installation "MOD_REWRITE REQUIRED" then you have to check few things:
You own a server?
If you own a server , edit a apache httpd.conf file and make sure mod_rewrite is not commented:
#LoadModule rewrite_module modules/mod_rewrite.so
And change it with:
LoadModule rewrite_module modules/mod_rewrite.so
Restart apache after changes.