Sorry for my late reply.
I don’t have an exact provision for calculating the ROI on OrangeHRM or on Open Source HR systems in general, but I can try and guide you towards a way of doing it.
When looking at ROI for an IT investment both the financial and the non-financial costs/ benefits should be considered.
On the financial side you can start consider the need for changing your operative system and the costs associated with this. OrangeHRM runs on both Windows and Linux. So as long as one of these is used in the company there will be no cost for changing operative system. Second there is the license cost which for OrangeHRM and open source software in general is 0$. Third you can consider the costs of setting up and running the system as well as monthly/annual cost of support and the need for updates.
On the non-financial side the following factors could be considered: Flexibility of the system. Possible impact on the operations and mission performance of the company and results (Here the human resource management). But these intangible factors are of course very hard to define and measure.
And last but not least make sure to evaluate everything using the proper time horizon as this can change the ROI considerably.
I hope I understood your question correctly and that my answer is useful.