1st Base Obstruction?

[MT 73]

Originally Posted by parrothead

So if he were receiving a throw, but is set up unnecisarily over say the whole bag causing runner to slow down, the throw didnt take him there, he just was standing there is that OB?

In OBR he cannot set up blocking the bag before the throw.
In other words he would have to be set up without blocking the runner's path but if F3 had to move in order to catch the ball and in doing so hindered the runner then that would not be obstruction.
And any collision resulting from such a play would be termed a "train wreck"
However many rule sets define obstruction as hindering the runner without having possession of the ball so there is no "In the act of fielding " provision.
In OBR your illustration would not be obstruction.
In others it would.
Check with the rule book that your League uses and read the definition of obstruction, which will state if there is an "In the act of fielding a thrown ball" provision.