Is it OK to leave my Ryobi 18 volt battery in the charger for long periods so it is charged fully when I need it?

Not necessarily.
Unless the rechargeable battery has been "conditioned", it may not even take a charge, let alone hold it. I had this problem with my Ryobi.
Begin by fully charging the battery(ies) per the manual. Then run the tool until the battery if depleted. Repeat this again. The battery should be able to maintain a charge longer. But it's not always best to just leave it in the charger. Consult the manual or their website for tech advice.

Cr, I have to agree, I don't think I would want to leave it on the charger all the time either. I would charge it until fully charged and remove from the charger.