Delete select (not all) photos from Galaxy S20FE without deleting from One Drive

[Hambone5]

My phone is full and I want to delete a LOT but not ALL of my pictures and videos to free up space.  My phone is synced with OneDrive.  I see an option to 'free up space' on my phone but that seems to select ALL of my content.  

I want ALL of my pics to remain on One Drive.  How do I select some of my pics/videos to delete from phone (leaving others on) but have the all live safe in the cloud??

[Orevix]

Click on the cloud icon in the top right, hit "Free up phone space" and then hit "Delete all". Then they'll be there but using no space at all. Then save the ones you don't want to delete afterwards

Otherwise there isn't a way, you'll have to go along and re-download them all.

An alternative is selecting the photos in one drive and making them 'available offline'

[Hambone5]

Thank you very much for this.  Could I ask a clarification?
When you say 'they'll be there', do you mean they will still be on my phone?? 
I don't understand how they 'can be there' and not take up space?

Finally I didn't understand 'then save the ones you don't want to delete afterwards'?]

To reiterate I want the ALL in the cloud but only 'some' left on my phone.  Purely for space reasons (on my phone).
Thank you so much for any clarifications......

[Orevix]

You can free up space meaning it will remove the main photo and only keep the thumbnail available in gallery so you can download the proper, un-compressed, higher quality photo. The photos will be stored on Onedrive but previewable in gallery.

Unfortunately you cannot do that. It will free up all of them. Sorry.