First, read the OGL carefully. If you're even a bit unclear on what the document legally means you're not yet ready to choose it as a license for your own work. If necessary, consult a lawyer. (Usually only necessary for work that involves significant money, however.)
With that said, basically yes. Others can use your OGL-licensed content in the exact same way that you can use WotC's OGL-licensed content. There are ways to make it difficult to reuse material, but without resorting to that kind of willful sabotage of your OGL-licensed work, everyone will be able to build upon it. “Everyone” includes WotC—they don't have any special rights to reuse OGL-licensed work that others don't have.
This was the intention of the OGL from the start in 2000—creating a base of developers who would enrich the D&D 3rd edition ecosystem without WotC having to either do much work themselves or spend much effort on legal oversight, since open-source communities tend to be self-propelled and self-regulated once they hit a certain critical mass.
On that note: building on others' work did happen with 3.x. It might not seem like it if you only look at 3rd-party rules additions, because not much building on each others' rules additions have happened—few developers seem to want to promote or be locked into relying on a competitor's rules additions when they can just write their own. Instead, the sharing happened mostly with monsters: the OGL statements of many 3.x-era bestiaries and (especially) adventures often have lengthy copyright statements covering all the OGL sources of monsters the work has drawn from.