{
    "topic": "openinterest",
    "ts": 1618820361552,
    "data": {
        "s": "PERP_BTC_USDT",  // symbol
        "oi": "936.37",        // open interest
        "ts": 1618820200000    // time of last update
    }
}

​
Topic: openinterest@{symbol}

Push interval: push every 1 second if open interest change and 10 seconds force update even if no change.
Subscribe to open interest. 
{
    "topic": "openinterest",
    "ts": 1618820361552,
    "data": {
        "s": "PERP_BTC_USDT",  // symbol
        "oi": "936.37",        // open interest
        "ts": 1618820200000    // time of last update
    }
}