Can you use loose nails in a Ryobi Nailer?

No.

Ryobi Nailers and Staplers are designed for specific fasteners compatible to these Nailers. 

Using incorrect sized fasteners/nails can be dangerous and will result in the tool jamming or breaking- Voiding the warranty. 

These Fasteners/nails do not need to be a specific brand, as long as they meet the specifications of the Nailer. 

As a reference: 
R18NL15-0 – 18V ONE+ Airstrike 15Ga DA finish Nailer. 
Uses 15GA DA angled brads 

R18GN18 – 18V ONE+ 18Ga Brad Nailer 
Uses 18Ga C1 Straight brads 

R16GN18 – 18V ONE+ 16Ga Brad Nailer 
Uses 16Ga C straight brads 

RFN1830X – 18V ONE+ HP Brushless Airstrike 30-34° Framing Nailer. 
Uses 10Ga angled framing nails (common framing nail)