O-uchi-gari countered by O-uchi-gaeshi.

Counter (Kaeshi-waza).

Techniques
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1. O-uchi-gari (大内刈), the major inner reap.
2. O-uchi-gaeshi (大内返), the major inner reversal.

About
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As your partner reaps from the inside, lifting the attacked leg and sweeping their reaping leg out with it is the major inner counter, the named answer.
This pairing is a named technique in its own right: O-uchi-gaeshi, https://wazabook.com/techniques/o-uchi-gaeshi/

Step by step (standard right-handed form)
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1. Their right leg hooks deep inside your left and starts to reap it away.
2. Their right leg is committed to the hook, so their left leg is carrying everything.
3. Before the reap is complete, use your left leg, the one they are hooking, to sweep their right leg out from under them.
4. They land on their back where they stood. Keep the sleeve and lower them under control.

Receiving it
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You are attacking, and before your reap is complete the leg you hooked sweeps your hooking leg away, leaving your weight on the standing leg alone. The fall is straight back and there is no time to post, so keep your chin tucked, let the held arm stay long, and slap with the free arm. A deep hook with no genuine off-balance in front of it is what gives them time to sweep it.

Demonstrations
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Kodokan: O-uchi-gaeshi https://www.youtube.com/watch?v=dCyZTXyjIXE
Efficient Judo: O-uchi-gaeshi - Demo https://www.youtube.com/watch?v=V5jQkLDww7g

VI judo notes
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You will feel their leg arrive inside yours before anything else happens, and sweeping it with the leg it is hooking is a movement you can make on that touch alone. The counter is leg on leg with the chest square, so nothing needs to be seen.

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