Optimizing an OAM file after Export with Imageoptim

It looks like that file was a .zip.oam – if you turn this option on Finder > Preferences > Advanced you’ll be able to see the full filename:

Make sure when you rezip your file, you are within the folder generated during the unzip process. Select:

Assets
config.xml

And then ‘compress’ those two items (not the parent folder), and then you also use the same name as the original .oam.