MOV does not play reliably on Windows, Android, smart TVs or in many upload forms. Opus sounds better than MP3 or AAC at low bitrates. MOV is at home in recording and editing on Apple devices; OPUS is the usual choice for voice and music at low bitrates. The OPUS contains the soundtrack only; the picture is dropped.
The defaults suit most files. Under Options you can choose the bitrate and trim the start and end. This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
The conversion uses your browser's own video and audio codecs (WebCodecs), with hardware acceleration where the device has it; the file is read from disk as it goes and can be up to 2 GB. Where the browser lacks a codec, the page falls back to the compatibility engine. OPUS is written at 128 kbps unless you choose another bitrate.