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Open interest is normally only visible as a live number on the ticker. This endpoint gives you its history: 
curl "https://api.polynode.dev/v3/perps/oi/btc?after=1783500000&limit=100&key=$POLYNODE_API_KEY"

{
  "instrument_id": 6,
  "symbol": "BTC-USD",
  "data": [
    {
      "timestamp": 1783571000000,
      "open_interest": "67.44542",
      "mark_price": "62010",
      "funding_rate": "0.0000125"
    }
  ],
  "more": true
}
Each sample carries the mark price and funding rate at that moment, so OI-vs-price studies need no joins. Samples are minute-resolution, newest first.
ParameterDescription
after / beforeTime window (Unix seconds or ms)
limitMax samples (default 500, up to 5000)
The series accrues from July 9, 2026 onward.