There is only one file type that uses the .OPDOWNLOAD extension:
Opera Partially Downloaded File
An OPDOWNLOAD file is a partially downloaded file created by the Opera web browser. It stores the contents of the file being downloaded from the browser. OPDOWNLOAD files are not meant to be opened since they store the contents of a file that has not been completely downloaded.
The .opdownload extension indicates that the download is incomplete. The extension is removed once the download is complete, and the file is then ready to be used.
If the file downloading from the Opera web browser is interrupted before it is finished, you may be able to still recover a portion of the file by deleting the ".opdownload" extension and trying to open it with a program that supports the file being downloaded.
NOTE: OPDOWNLOAD files are similar to .CRDOWNLOAD files used by Google Chrome and to .DOWNLOAD files, which are used by Mozilla Firefox and Apple Safari.
Updated: October 6, 2019