Once upon a time there was a difference between "Lead" and "Iso". That difference pretty much summarizes the difference between "man" and "gap" blocking. The summary, in today's lingo, is often that there is no difference. Needless to say, the terms are fairly interchangeable today, but we will call it double lead because it is technically a little more accurate.
"O" is something you often here with "Power O". You'll also hear things occasionally like "Power G" or "GO Sweep". The "G" and the "O" determine the pulling guard. "G" means the playside guard whereas "O" means the opposite guard.
So "Double Lead O" will be two lead blockers and a backside pulling guard. The play call will also include strength of the formation, the formation itself (including the unbalanced line), the motion, the intended ball carrier (denoted by a number), the intended hole, and the play call. It'll end up something like (but probably completely different) than "Right Rhino I Over Zap 46 Double Lead O". "Right" means TE to the right, "Rhino" is the formation (Wing lined up just outside of TE), "I" is the backfield formation (I-formation), "Over" is flipping the OT over, "Zap" is a motion that goes across the formation then comes back from the Z-receiver (wing here), "4" is the tailback, "6" is the hole the RB is trying to run through, "Double Lead O" is the blocking system. Very little of this will translate to other systems. It's a problem in football: there is no consensus on terminology on a lot of things, not even close. On other things, there is, which makes it even more confusing.
Here is how the play is blocked against an over front.
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To continue reading about this play, and how this play will adjust to other defensive fronts, follow the link to Maize n Brew.