Osh is a genuinely different Kyrgyzstan from Karakol and Issyk-Kul — older, more Central Asian in feel, and centered on Sulaiman-Too, a UNESCO-listed sacred mountain that's been a pilgrimage site for over a thousand years.
The mountain itself is walkable in a couple of hours, with a small museum built into the rock and views over the whole city from the top. Osh's bazaar is one of the oldest and largest in Central Asia, and worth as much time as the mountain, if not more.
Whether it's worth the trip depends on your timeline — it's a separate flight or a long overland journey from Bishkek/Karakol, not a quick add-on. For travelers with 2+ weeks in the country and an interest in history beyond mountain scenery, it's a genuinely worthwhile detour. For a shorter first trip focused on Issyk-Kul and Karakol, it's reasonable to save for a return visit.
Zoomy