can i upgrade straight from OS Tiger to OS El Capitan

Also 10.5 and later will not allow you to utilize Classic applications on an iMac G5. If you had another PowerPC Mac, you might be able to dual boot into Mac OS 9 for older applications:

https://discussions.apple.com/docs/DOC-1757

And then you have another barrier. Mac OS X 10.7 and later get rid of Rosetta, which means all your older PowerPC applications become obsolete, unless you run 10.6 Server in Parellels. So the question becomes is what is your application need?

This tip should help you determine how far you might want to go: https://discussions.apple.com/docs/DOC-6271

It is not worth the trouble/expense/hassle to upgrade to Leopard at this stage in the game unless you have a very specific reason to do so. There's perhaps a small gain in the number of software packages you could run under Leopard vs. Tiger but even those are several versions out of date compared to the newest. There's a glass wall between the G series PPC computer and the Intel Macs and you can get a low end Intel Mac (which albeit itself maybe somewhat out of date) for a couple of hundred $.

If the Mac is as indicated in your profile (iMac G5), it can not be upgraded beyond Leopard (10.5.8)

If you plan on using at all with 10.4 or 10.5, give TenFourFox a try.

http://www.floodgap.com/software/tenfourfox/