Hi,
In Odoo, odoo online, refers to saas service provided by odoo, which is odoo enterprise edition. So if you are using, odoo online and free plan, the instance is odoo enterprise edition (not community edition)

Regarding to the installation of app, other than data files app, odoo online wont support. So can you clarify which app you have installed, preferably a link to app in the store.

Thanks