[CecilOne]

Top of the first inning, R1 on 2B, 2 outs. B4 hit a squirrelly little ground ball to the pitcher's right. F1 fielded it and threw to F3, but the throw was down by the BR's feet. Between F3 reaching for the ball and B4 trying to get to 1B, there was a collision and both players went down. The ball caromed off the players to the fence. As B4 tried to get up to go to 2B, she was OBS by F3 trying to chase down the loose ball.

At that point I called OBS. R1 rounded third and headed for home when she saw the ball go to the fence.

Once B4 got up, she started to go to 2B with the coach yelling at her to "Go! Go! Go!" By this time F3 had retrieved the ball and started back toward the infield. B4 thought better of trying to go to 2B and instead headed back to 1B. Now it was a race to see who'd get there first.

I had come inside once the ball got past F3 and had taken a couple of steps toward 2B when B4 initially thought about going. Now I had to try to get closer to 1B for the play there.

Both players dove for the base. I clearly did not have the preferred 90-degree angle and was probably about 15' away from 1B when I called the runner safe.

By this time, R1 had scored. I called time out, and because I had the OBS call, announced that I had OBS on F3, but that B4 would remain at first base.
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It looks like there were 2 OBS, although the before 1st is not clear; collisions are not always OBS or INT. The OBS at/after 1st is clear as called. That OBS occurred.
We apply the result of an OBS based on where we judge the runner would have ended up without the OBS. With the ball quickly retrieved by F3, there appears no chance the runner could reach 2nd safely. The runner is protected from being out between 1st and 2nd because of the later OBS; therefore is placed at 1st.