1. Develope an API for your external application. Your API must read data and write it if is necessary.

2. Embed the code inside the desire modules for integrate the other app in odoo 

Example update price of products from other app. When odoo "open" the data of some product, first read the "api-from-other-app", compare prices and update it.

Hi,

For such integrations, normally we will design our own tool in Odoo. Suppose for example if you need to read data from MsSQL db and create records in Odoo, you have to built such a tool.

A module which utilizes the python libraries like pymssql and connect to the other software fetch data and using odoo orm methods, create records in Odoo tables.

Thanks