Not a Lawfag, but been pondering why no oath. Not sure which side decided this, would assume this was decision made by members of the Judiciary Committee.
At this point, Comey lied under oath so many times and while /ourguys run down the clock and cross t's and dot i's (as another Anon put it) bringing Comey back to testify, but not under oath, may be a message. The message may be that it's not worth asking Comey to take the oath because they knew he's lied under oath so much anyways - it would be a huge joke to go through the motion. But hey, let's just get him here and see what the fucker says now at this point. See him squirm. Ask him the difficult questions when we know, he knows we know, and while glibly not asking him to raise his traitorous right hand.