Hi,
In Odoo online, if you need to trigger a function from button you can do as follows:

* create a server action and write your logic here or call any existing function from here.
​For eg: if you need to trigger action_confirm function in sale order model, create a server action with model as sale.order and action to do as execute python code and in the code section add record.action_confirm().  You can iterate it over for record in records to handle multiple records

* once server action is created copy its ID and in your button, set the type as action and in the name give this ID

This is how you can trigger a function from a button in odoo online.

Thanks