i swim all the time with a tampon. just use a more absorbant one if you are heading to teh pool, and as soon as you get out 9well, within 30 minutes) i woudl go and change it. but there is nothing wrong with it! enjoy the party!
PS. i use OB as well! always worked and like them better than playtex sport. also have used the cup, but tricky sometimes to get it in correctly! but when you do, fab!