I emptied my OSP last year without having to pay any extra taxes. Here's how it works.
Suppose the value of your OSP is $150K and that $100K of it is money that you contributed to your OSP with after tax dollars. Let's suppose that the other $50K is gains due to your investment choices. You may withdraw all of the money that you contributed (the $100K) with no tax consequences. Vanguard will send you a check.
You can also roll the gains ($50K, in my example) into an IRA or leave it where it is. The penalties apply when you try to do something with the gains other than roll it over to another IRA.
This worked for calendar year 2010. It would be wise to call Vanguard and ask what the current rules are.