You were able to buy tickets JUST for the park while the Olympics were on. This gave you access to the park but no actual events of course.
Buying ticket for an event WITHIN the Olympic park automatically gave you access to the park of course. Tickets to a NON Olympic Park event (like Wembley) did NOT give you access to the park.
The same applies for the Paralympics, though I believe all the Olympic Park only tickets have already been sold.
If you want to check, go to the London 2012 web site, and search for tickets. Select "Olympic Park" as the venue but dont chose any actual event.
This will list all the tickets left that will get you into the park.
Note AFTER the Olympics the park will close for more than a year while they take down temporary building, build new houses on the site and so on.