What does o mean on iPhone 4?

So I have sprint, and occasionally instead of the 3G symbol o comes up. What does it mean? Or stand for?

Sprint voice and (slower data) runs on CDMA/1xRTT. Faster data is on 1xEvDO (3G). That "o" says that the slower 2G network is available. This usually appears for a little after power cycling - it de-registered when powered down and needs to re-register at power up. Either that or the 1x-EvDO network is not available for some reason… (1x-EvDO = 1xRTT-Evolution Data Only… This is being replaced by the 4G LTE network.)

In Safari, click Bookmarks at the bottom.
Click iPhone User Guide.
Click iPhone at a Glance.
Scroll down, Click Status icons.
Scroll down to "o".

GPRS aka 1G.

o is 1G (GPRS)
E is 2G (EDGE)
3G is 3G
4G is 4G (HSPA)*
LTE is 4G (LTE)

*usually only on AT&T models