How to extend metadata in ojs 2.x

HI am trying to find a way to extend article metadata. I need one or two custom fields, and I want to be able to access those data trough oai nlm, like in this journal:

I also need references listed as metadata.

As a bonus, it would be nice to show that data on article page, but it is not priority at this moment.

I am using ojs 2.4.8, and still not planning to upgrade to 3.x

i have some php coding experience and any suggestion or hint will be appreciated. :slight_smile:

sorry for bad english