You can accomplish this with a combination of Apache rewrite rules (mod_rewrite) and OJS configuration.
The rewrite rules will take incoming requests for the URL you want and map it to the URL OJS expects.
Then, when generating a response, OJS needs to be properly configured so that any URLs it generates are also of the form you want to use. You can configure this with the base_url directives in config.inc.php. Note that you'll probably need to enter a few base_url options to get this working. Once you get mod_rewrite configured, feel free to ask me for help with this.
I don't have any specific experience with Apache rewriting rules, but there is a lot of documentation and many examples online, and the rule you want to write should be fairly straight-forward. If you do manage to find a good rule, please post it here so that other users can use it as a reference.
FYI, you may be able to use mod_alias instead of mod_rewrite when configuring Apache; this is easier to configure but I can't guarantee it'll be able to mask the index.php part of the URL.
Open Journal Systems Team