A scantool would show you alternator output, as would a voltmeter. The problem is that at idle with a voltmeter, you're not really seeing the big picture. 14 volts is in the range. Assuming your battery is fully charged.
Let'* think about changing the O2. It'* got to be how old now? Basic rule of thumb is 50-60k miles or any time your fuel mileage drops, or fails to achieve what it should, or a rich condition.
Even if it doesn't fix the problem, it probably is due at this point. Time to order one, and in the meantime, we can make sure you've got enough spark voltage. How about a voltage check at idle, warmed up, after driving (to make sure the battery is fully charged and not demanding too much), as well as a coil pack resistance check. We can cover the most likely bases in the next few days that way.