It's rare as hell, but you can commission directly at higher ranks if you have the required experience and credentials and there is sufficient need in the service. The CO of the dental unit at Fort Hood that did my gum grafts 14 years ago direct commissioned at O6.

As there isn't really any civilian equivalent to combat branches of service, however, they won't direct commission anyone into the infrantry at that level, sure, at least not since the civil war era.