"Not Ready " response from emissions readiness monitor for the catalyst converter?

I took my 2007 Subaru Forester for an
emissions test a week ago. The test
showed an overall "fail" because of two
codes that popped up for the O2 sensor, as
well as the OBD system/readiness monitor
showing "not ready" for the O2 sensor and
the catalyst converter. The check engine
light was on as well. I got the O2 sensor
replaced the day after, and the check engine
light has not been on since. However, when
I returned to get the emissions retest, they
told me that the readiness monitor was still
showing as "not ready" for the catalyst
converter. There was no guidance on fixing
the problem, just that it needed to be fixed. I
then visited 3 auto parts stores for opinions,
and got the following recommendations: 1)
the sensor may be showing 'not ready' after
the O2 sensor was replaced, as the
computer may have been reset (the battery
was unhooked) and I may need to drive the
car an additional 60 mikes to get the
computer to register correctly; 2) I may
need to replace the converter, however
there's no way to confirm. The stores did
the computer and verified that absolutely no
codes are generating. Has anyone
experienced this issue before who could
offer some insight on the best course of
action from  this point?