Here’s a quick tutorial on how to show modified post date in Oxygen.
Single Posts
Edit the Oxygen template for your single posts and add a text element where you want to display the post’s modified date.
Double click the text and click on “Insert Data” at the top.
Scroll down and select “PHP Function Return value”, then paste the_modified_date inside the “Function Name” field. Save the template.
It may show “NO DATA FOR FIELD” in the Oxygen editor but does render correctly on the frontend.
Archives
Edit the Oxygen template for your archives.
Assuming that the posts are being displayed via a Easy Posts element, select the element, then Templates > Template PHP.
Find a line similar to
<?php the_time(get_option('date_format')); ?>
and replace it with
<?php the_modified_date(); ?>
Reference: https://codex.wordpress.org/Function_Reference/the_modified_date