Maximo Open Forum

Maximo's REST API uses oslc/os, how to define a method to return your own json data

Generally speaking, you want to use the API as much as possible. There is a ton of functionality built around it to support concepts like pagination, limiting MBOs in memory, multiple data formats (XML, JSON, CSV, etc.), caching, and so many more concepts. Customers can define their own object structures to get data so if it's something that is not available out of the box, you can create it. 

If your requirements are too unique to be able to use the existing framework, there are a lot of different ways you can do this. But a common one is people use the automation script handler like:

IBM Maximo Autoscripting Guide – Invoking script through Automation Script Handler

The example is an action but you can set the responseBody to return data back to the user.