Photos library

But photos on Omnidisksweeper is showing two libraries

one for 90.2gb and the other 78.1 migrated.

When you migrate an iPhoto Library or Aperture Library to Photos for Mac, Photos will preserve the original library and rename it to iPhoto Library.migratedphotolibrary. And it will create a new Photos Library.photoslibrary.

Both libraries will be showing approximately the same size in the Finder, but they are sharing the storage for the image files by hard links. So they are using much less storage than the sum of the size of the two libraries.

See these documents:

Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

and

Six Colors: The (hard) link between Photos and iPhoto

You can delete the iPhoto Library.migratedphotolibrary from your internal drive and back it up somewhere, if you no longer use iPhoto, but it will probably release less storage than its size, because the image files inside are shared with the Photos Library.

If I delete the migrated I won't lose the photos library will I?

No, all photos are in your Photos Library as well.

A hard link is creating a new file, but it is stored on the same disk blocks as the original file. And OS X is counting, how many linked files are using the same storage. The storage will only be released, when the last link is gone.

Thanks, but then why does my mac have no storage?

What else are you using that might be using storage? iMovie? iTunes? iCloud Drive will also need internal storage. All iCloud Drive files have shadow copies in your user library in your home folder.

How large is your Home folder?