Hi @Dom
OneNote is predominantly used as a note-taking application, its strengths lie in its simplicity. Compared with Word, Excel, etc., there are fewer options for table formatting.
Currently we are unable to set the border thickness. Please refer to the documentation for the options that can be set: https://support.microsoft.com/en-us/office/format-a-table-03a23b1f-5fdd-40aa-b677-ce32ef47f9af
Sometimes, to improve the look of a table on a page, you might format a table to resemble a simple spreadsheet.
Then you may format table in Excel. For more details, please refer to: https://support.microsoft.com/en-us/office/convert-a-onenote-table-to-an-excel-worksheet-f693d0ec-aad7-4441-9596-e5e1e641935d
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.