I actually had a pretty clear cut case of OBS this past weekend at a 16U PONY Tourney:
Throw over to 3rd to force a runner gets away from 3B,both 3B and C go chasing after the ball. The P,meanwhile had started to go chasing after the ball,but saw the baserunner turning for home...so....she decides to go cover HP..running directly down the baseline..about five feet in front of the baserunner the whole way-no question in MY mind that was impeding the runner there...THEN the P seals the deal by doing a perfect spin...thereby blocking the plate w/o the ball forcing the BR to make a short short slide..the arrival sequence at home being something like: P..then BR and ball a millisecond after the BR...I had NO problem calling OBS,and awarding home....the defensive coach was another matter,however...