Is there anything like OneTab available for Safari?
Here is a native way of doing it without the use of any extensions.

In Safari, under the "Bookmarks" menu, there is an option for you to "Add Bookmarks for These X Tabs...". If you click on it, you will be prompted to save these tabs in the current windows as bookmarks. You can assign them to a Folder and give it a Session Name.



When you do want to resume the session, you simply go to the bookmark folder, right click on it and select "Open in New Tabs".



The beauty of this methods is that, if you have iCloud sync enabled, you will be able to resume your saved sessions across all your Apple devices.