Isn't the Online part here about getting results immediately after query, as opposed to overnight batch reports? So if you don't completely overwhelm DuckDB with writes, it still qualifies. The quality you're describing is something like "realtime analytics", and is a whole another category: Clickhouse doesn't qualify (batching updates, merging etc. — but it's clearly OLAP), Druid does.

Huh yeah looks like I was totally wrong about what online meant. So yeah DuckDB is OLAP. Not that anyone was asking me in the first place. Carry on :)

ClickHouse is the market leader in real-time analytics so it's an interesting take that you don't think it qualifies.

For certain definition of realtime, certainly (as would any system with bounded ingestion latency), but it’s not low-latency streaming realtime. Tens of seconds or more can pass before new data becomes visible in queries in normal operation. There’s batching, there’s merging, and its overall architecture prioritizes throughput over latency.