The only declarations required by the OGL is Section 8 that you clearly mark OGC used, if any, in your work and that you use the notice from Section 2.
What it says is this:
"8. Identification: If you distribute Open Game Content You must clearly indicate which portions of the work that you are distributing are Open Game Content."
If I can't tell at a glance what is OGC and what is not, you violate the license. If you only make an OGC declaration and not a PI one, then I guess you aren't clearly identifying it and nothing is PI, except those things already specified in the license itself.
Which does not include transforming OGC sourced material into PI.
What you are describing is not a logical possibility. The OGL doesn't "transform" anything. It is a license to produce derivative works. You can't make something PI that is verbatim OGC. However, you can make entirely PI works that are entirely derivative.
1. "(g) "Use", "Used" or "Using" means to use, Distribute, copy, edit, format, modify, translate and otherwise
create Derivative Material of Open Game Content.
"(d)"Open Game Content" means the game mechanic and includes the methods, procedures, processes and routines to the extent such content does not embody the Product Identity and is an enhancement over the prior art and any additional content clearly identified as Open Game Content by the Contributor, and means any work covered by this License, including translations and derivative works under copyright law, but
specifically excludes Product Identity.
"(e) "Product Identity" means product and product line names, logos and identifying marks including trade dress; artifacts; creatures characters; stories, storylines, plots, thematic elements, dialogue, incidents, language, artwork, symbols, designs, depictions, likenesses, formats, poses, concepts, themes and graphic, photographic and other visual or audio representations; names and descriptions of characters, spells, enchantments, personalities, teams, personas, likenesses and special abilities; places, locations, environments, creatures, equipment, magical or supernatural abilities or effects, logos, symbols, or graphic designs; and any other trademark or registered trademark clearly identified as Product identity by the owner of the Product Identity, and which specifically excludes the Open Game Content; (f) "
"4. Grant and Consideration: In consideration for agreeing to use this License, the Contributors grant You a perpetual, worldwide, royalty-free, non-exclusive license with the exact terms of this License to
Use, the Open Game Content."
The "derivative works" referenced above clearly refers to "any work covered by this License, including ..." There are some thorny issues in the license, but it comes down to this: there is nothing preventing you from taking OGC, altering it, and declaring it PI.
A kosher example would be taking an ordinary +1 longsword and renaming it "Zarg's Blade." A complete non-starter would be to take existing OGC, make trivial changes that are not "enhancement over the prior art" and try to call it PI. In practice, since OGC has to be clearly labeled, a publisher can retain control over original concepts simply by not declaring them OGC (which makes them not). Aside from that, there are many interesting cases.
For instance, Green Ronin did not make power points OGC in their M&M, but it's not clear to me that points used to buy powers and abilities are an enhancement over prior art (making them copyrightable) or if they are, one of the categories of PI (making them protectable through the license). Arguably, they are "special abilities." The best protection for Green Ronin is the inability to declare compability without getting a license from them.