Have you ever pushed your limit on SEO efforts, and experienced the OOP? On November 15, 2003, the Google SERPs were dramatically altered by Hurricane Florida, also known as Over Optimization Penalty. Google has increased the threshold for triggering the OOP since, and SERPs are back to pre-Florida level.

One of SEO technique well known for OOP target is over usage of anchor text. For example, you decided to change the navigation on your site to include a keyword, say "Keyword Home" instead of "Home". After a few days, you lose search engine rankings. You change navigator menus back to original, without the keyword, and the lost rank comes back.

Google does not want a site to rank well overnight. If you start a new site, you can easily build search engine optimized pages with good backlinks and your site will be placed in a sandbox for several months before slowly rising to the top.

On the other hand, if you have an existing site and decided to optimize your site with the seo effort. The new pages are fully optimized, but now the OOP comes into play. The site does not rank well and gets held back for several months before slowing rising to the top. This isn't proven, but OOP seems to play bigger role in more competitive keywords -- money making keywords.

The OOP exists for the same reason Sandbox exists, and delays a site from rank well overnight. This might have some affect on Google Adwords sales, as new entrants will likely buy adwords for the keywords until its rank rises.