An open GOP is a Group of Pictures whose final B-frames reference the I-frame of the next GOP. Where a closed-gop is fully self-contained (no frame in it depends on anything outside), an open GOP lets the last few B-frames "reach forward" into the start of the next GOP to find better predictions. The result: slightly smaller files (typically 2–5 % savings) because B-frames have more reference frames to choose from.
The trade-off is that an open GOP is not independently playable from its start. If you tried to start playback at the first frame of an open GOP, the final B-frames at the end couldn't be decoded yet because they need the next GOP's I-frame, which isn't available. For a standalone download or a continuous playback this doesn't matter — the player keeps moving forward and finds the reference when it arrives. But for adaptive streaming (HLS, DASH), where each segment must be independently decodable from the start, open GOP boundaries cause visible glitches when the player switches between bitrate renditions mid-stream.
For a product team, the choice is essentially: closed GOP for adaptive streaming, open GOP only when you don't need clean segment boundaries. The 2–5 % bitrate savings of open GOP simply don't justify the playback issues for VOD streaming. Most encoders force closed GOPs when you configure HLS or DASH output. Open GOP survives in non-segmented contexts: progressive downloads, archival masters, broadcast contribution feeds where the entire bitstream will be re-encoded downstream anyway.
