As of version 3.6, there seems to be a bug in Oxygen where an element's background image coming from a ACF field selected at responsive breakpoints does not appear on the front end.
This members-only tutorial provides a workaround to fix this by injecting custom CSS at the end of Oxygen's stylesheet.
If you have not already, create a Advanced Custom Fields field group having two Image-type custom fields labelled say, "Desktop Background" and "Mobile Background". Set the Return Format to Image URL. Set the group to appear on your desired post type, say, Page for example.
In the Oxygen editor add the element for which you would like to set different background images at different media queries.
At Advanced > Background set Background Repeat to no-repeat and Left and Top values to 50% each.