I have a Dell Inspiron 1500 gaming laptop [https://www.bestbuy.com/site/dell-inspiron-15-6-laptop-intel-core-i7-16gb-memory-nvidia-geforce-gtx-1060-1tb-hdd-256gb-ssd-matte-black/6162034.p?skuId=6162034] and for some reason my OS (SSD) randomly filled up and only has like 18GB of free space, the only thing I have on it is GTA. Any reason why all of the space would be taken up so fast? My laptop runs LSPDFR great but the OS space is concerning me on why I randomly have only 18gb left of free space.

Try removing any old temporary files:

https://support.microsoft.com/en-us/help/4028075/windows-delete-your-previous-version-of-windows

 

you might also have lots of restore points saved, They can be found the same way as detailed in my above link, but in the system and reserved section.

Yeah, if you recently updated to a new Windows 10 build (within 10 days as it is supposed to automatically delete), you'll have a ~30GB folder called windows.old containing the old version (just further explanation of the first part of the above post).