Is there a linux OS that can be loaded entirely into RAM?

I think all distros can be run from RAM, you only need to do some changes. Read this link

You need to have in mind that any changes (updates, and the like) that you do to the filesystem that is in memmory are lost when you turn of your machine, so you need to stablish a mechanism to update your HD with those changes BEFORE the system goes down, that will delay the shutting down of the system.

Ubuntu can run on RAM, but it requires some manual changes:

https://wiki.ubuntu.com/BootToRAM

Puppy linux is a distro that can be and is designed to run from RAM.

https://en.m.wikipedia.org/wiki/Puppy_Linux