I have a ryobi 18V battery and the only charger I have is a ryobi 14.4V. Can I charge the 18V battery on that charger?

hi Terry.
you could try homebase.or ebay will have them.
or this link should show you where you can buy them.
battery charger for ozito drill Google Search

Ryobi P115 is an 18V battery charger. However, Ryobi Manual did not mention the current rating of P115. if the current rating of Ryobi P115 is 50% higher than your 18V Lithium Battery it will charge. but if it's less than or equal, it will not.

I just went through this with Porter Cable 18V but I can't say it is the same for Ryobi. On the PC, all 18V batteries are interchangable on the tools, but the charging needs are different. A dedicated NiCad charger can't shouldn't be used for liOn but the later chargers will work for either battery.
Ryobi offers an 18V upgrade:
RYOBI Tools

That is true the One+ 18V Li-Ion battery packs will work in any One+ 18V Ryobi tool. However the Lithium ion batteries will not charge in the original P110 NiCd charger. You will need one of the the new series Ryobi One+ 18V "dual chemistry" chargers for that. The will charge both types of battery packs.

NO! Never charge lithium batteries in a charger designed for nickle based batteries like the NiHi your old charger was meant for. Ryobi did design the new Lithium charger to be backwards compatible so you can charge your older 18V NiHi batteries in the Lithium 18V charger.
Also note, NEVER miss-match the voltage, for example, make sure you are not trying to charge 12V batteries in an 18V charger unless the manufacturer has specifically stated that the charger is capable of multiple voltages.

No, absolutely not! But it won't charge it either.

Absolutely not. The risk of damage to the battery is only a minor concern compared to the fire hazard you would be creating.

Does it not hold a charge, or will the charge not indicate "fast charge"? I had to apply a initial jump charge on the battery to get it up to 20 volts, then I put it in the Ryobi Charger, and it started charging it.

For others with the same problem (flashing red light won't go off)...make sure you FIRMLY PUSH the battery down onto the charger and engage the latches. Simply resting the battery on the charger will not work.