Please go through the following checklist in the given order, without missing any paragraphs, the solutions are presented in the order of probability :

1) Did you flush all cache, log out and log back in to Magento Admin?

2) check
System>Configuration>Advanced>"Disable module output",
is the module output not disabled ?
Please do not use that Admin feature to disable module output for any modules,
it may lead to some hard to detect issues,
instead use /app/etc/modules directory for disabling modules.

3) If there have been any other 3rd party One Step Checkout modules
installed on your system, please uninstall all of them COMPLETELY. If
necessary, contact their module support for uninstall instructions.
Especially pay attention to files in theme paths:
app/design/frontend/[your theme folder]/onestepcheckout
app/design/frontend/[your theme]/layout/onestepcheckout.xml
skin/frontend/[your theme]/onestepcheckout/

Disable all matching leftover file and folder names like
onestepcheckout.xml and onestepcheckout/

In linux/unix, use
grep 'onestepcheckout*' * -ris
and on Windows based system use text search in folders
to see if there are any leftover files on your system that should not
be there and may break onestepcheckout.com module.