Let me try again, breaking this down into the individual points that I'm raising:
1) The title of Section 3 is "Offer and Acceptance." This seems to imply that the OGL as written contains an offer.
2) The rest of Section 3 reads, "By Using the Open Game Content You indicate Your acceptance of the terms of this License." This seems to state how one accepts the aforementioned offer.
3) More specifically, you accept the offer by "Using" the Open Game Content.
4) "Using" is defined in Section 1(g) as, "to use, Distribute, copy, edit, format, modify, translate and otherwise create Derivative Material of Open Game Content." So
creating Derivative Material that didn't previously exist counts as Usage.
5) Compounding this, Section 1(b) defines "Derivative Material" as "copyrighted material including derivative works and translations (including into other computer languages), potation, modification, correction, addition, extension, upgrade, improvement, compilation, abridgment or other form in which an existing work may be recast, transformed or adapted." Because this says "
including derivative works [et al]" rather than "
only derivative works" (or something similar), that seems to suggest that it doesn't have to be
only derivative works of copyrighted material;
any copyrighted material will do, including original material.
6) As such, "Derivative Material" of Open Game Content includes original Open Game Content (as counterintuitive as that sounds).
7) Hence, creating original work which is subsequently released as Open Game Content (and identified as such per Section 8) under the Open Game License constitutes acceptance of the offer mentioned in the title of Section 3.
8) This holds true even if no Open Game Content from any other OGL product is used in your OGL product (in which case the only things in your Section 15 are your product's copyright notice and the OGL copyright notice).
9) With all of that determined, the only question left is who made the offer which you're accepting. I'm not sure there's any other answer than WotC.
Hopefully that makes what I'm trying to outline clearer.