O-5 isn't that much power - as a battalion commander, that's leading about 300-1,000 soldiers. They wouldn't necessarily be leading troops, but instead be in advisory roles where the rank might have them be taken a little more seriously by flag officers than if they were commissioned as a company grade officer.
>>snerbl+(OP)
Direction commission officers aren't line-officers (AFAIK), so they are don't command in the way you're describing. That assumes doctor/lawyer/nurse though...