Hi Sakis,
the scheduled tasks (which are automatically created (unpublished)) for you are replacing the 'old' crons: they do exactly the same and are still configured in the automation tab of the component.
In the next major version the old cron functionality will be removed, then only the scheduled tasks will work.
When you install the new version you will be notified of the new scheduled tasks (if not enabled), you will also be notified if you are still using the old cronjobs: this to provide a move over to this new functionality without leaving you in the dark somehow.
There are basiscally two ways to use the Core Joomla 'Lazy scheduler', out of the box it runs its tasks on the session of a visitor.
This means that timing of the cronjobs depends on a visitor visiting your site, also when the cronjob e.g. takes 10 seconds to complete the visitor will have this 10 second load.
So although this works, this is not optimal, especially for an ecommerce site.
So it is best to configure the Joomla core lazy scheduler as a webcron (it is defined in the options of the scheduler). When you do that a unique token is generated and you will be provided an URL that when visited will start the scheduled tasks component on your site.
This URL you need to set as a crontask on your server (or somewhere else), just like you now have set the two cronjobs for ochSubscriptions. You can set it to run every minute or 5 minutes.
Here is some more in-depth information from Akeeba which also uses the scheduled tasks:
www.akeeba.com/documentation/admin-tools...joomlascheduled.html 
