O-soto-gari countered by O-soto-gaeshi.

Counter (Kaeshi-waza).

Techniques
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1. O-soto-gari (大外刈), the major outer reap.
2. O-soto-gaeshi (大外返), the major outer reap counter.

About
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The direct answer to a major outer reap: the reaping leg is off the mat and carrying no weight, so reaping it in turn takes their only remaining support.
This pairing is a named technique in its own right: O-soto-gaeshi, https://wazabook.com/techniques/o-soto-gaeshi/

Step by step (standard right-handed form)
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1. Feel the back of their leg arrive behind yours as they attempt O-soto-gari.
2. Bend your attacked leg and drive your hips forward into them so the reap cannot straighten it.
3. Keep both grips and pull their chest onto your shoulder line so their weight comes forward.
4. Their reaping leg is still in the air: swing your own leg past it and reap the back of their supporting leg.
5. Drive your shoulders forward and down over the reap.
6. They fall backwards; keep the sleeve and stay upright over them.

Receiving it
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You are the attacker, and the reap arrives to find their leg bent and their hips driving forward into you, which leaves your own reaping leg in the air with nothing to come back to. Their leg then swings past it and takes your supporting leg. It is a fast backward fall from a full commitment. Keep your chin on your chest, do not reach behind you, and slap hard with your free arm as your back meets the mat.

Demonstrations
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Mike Swain: 52:57 Osoto-gari countered by Osoto-gaeshi https://www.youtube.com/watch?v=_cCAytbmk3g&t=3177s

VI judo notes
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The counter is decided at the moment of contact behind your leg, which needs no sight: bend the attacked leg and drive your hips forward before the reap can straighten it. Keep both grips and pull their chest onto your shoulder line so their weight comes forward as your own leg swings. Reap the leg they are standing on, not the one that is already travelling.

From Waza Book: https://wazabook.com/counters/o-soto-gari-countered-by-o-soto-gaeshi.txt