I believe the earliest releases of OmniFocus did run on a PowerPC with 10.4.
I can't seem to find a public download link, but if you contact our Support folks directly, they'll let you know if anything's available:
omnifocus@omnigroup.com
(But please understand that if you encounter bugs on that older version, our response will most likely be "sorry, we've fixed that in the version that requires a newer OS.")