To uninstall OneStepCheckout you need to remove following files from your server:

app/etc/modules/IdevOneStepCheckout.xml app/locale/*/IdevOneStepCheckout.csv app/code/local/Idev/ app/design/frontend/base/default/layout/onestepcheckout.xml app/design/frontend/base/default/template/onestepcheckout  skin/frontend/base/default/onestepcheckout

and any customized template or layout file that you have copied under your custom theme/skin folders.

After uninstalling extensions please clear all caches .