A quick orientation to Japan's benchmark index options market, and the free official data this site turns into daily charts.
Strikes with heavy open interest often act as reference levels. A large put wall below spot marks where hedging demand concentrated; SQ week tends to gravitate toward high-OI strikes. Combined with the Nikkei VI (Japan's volatility index) you get a quick regime read: walls close + VI low = pinned market; walls broken + VI spiking = trend risk.
This is the most common error, and Nikkei options make it easy to commit.
In early August 2026, the single largest open interest in the entire Nikkei options chain was the 30,000 put, at roughly 5,600 contracts. The Nikkei was trading near 65,600 — so that strike sat 53% below spot. Over eight sessions the position moved from 5,592 to 5,623 contracts: essentially dead. It is legacy or deep tail protection, and it has nothing to do with current price action.
Meanwhile the 70,000 call, about 6% above spot, moved from 4,490 to 4,910 contracts in the same window. Smaller, but alive and reachable. That is the strike that matters.
For this reason we restrict "walls" to strikes within ±10% of spot: the highest call open interest above spot, and the highest put open interest below it.
1. The back month can be larger than the front month. On 7 August 2026, one week before the August expiry, August open interest stood at 169,955 contracts while September held 189,160. September is a Major SQ (futures expire alongside options), so quarterly hedges concentrate there. Looking only at the front month will misread where the market's attention is.
2. Open interest rises into expiry rather than winding down. The same August series went from 137,311 contracts on 17 July to 169,955 on 7 August — up 24% in three weeks. Short-dated options are cheap and responsive, so short-term flow concentrates into them. The position does not decay away; it accumulates and then vanishes at SQ, which is why the supply-demand picture changes abruptly around expiry.
The Nikkei put/call ratio normally sits above 1.0 — our measured average is 1.57 for large contracts — because institutional put hedging is structural. Judge it against its own range, not against 1.0.
Mini contracts behave differently: the same period averaged 0.91, consistently below the large-contract ratio. Large is institutional hedging; mini carries more retail upside-seeking flow. The gap between them is itself informative.
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