Document

What is a .ots file?

A .ots file is an OpenDocument Spreadsheet (version 1.0). This is a document format, so it holds text and layout meant to be read. Confusingly, this extension has been used by 3 different formats over the years — they're all listed below.

This extension has been used by more than one format

3 different formats have claimed .ots over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .ots file

Try LibreOffice (free, Windows/Mac/Linux) — it opens an enormous range of current and historic document formats.
Microsoft Word and Google Docs handle most mainstream document types.
If nothing opens it, open it in a text editor: many older document formats have readable text inside.

The registry records this extension against OpenDocument Spreadsheet, OpenDocument Spreadsheet, OpenDocument Spreadsheet, so software from that family is the most likely match.

How to convert a .ots file

Once it opens in LibreOffice or Word, Save As .docx or .pdf to get it into a format you can use anywhere.

MIME type

When a .ots file travels over the web, servers label it with this MIME type:

application/vnd.oasis.opendocument.spreadsheet

What's a MIME type?

The technical record

For archivists, developers and the merely curious: these are the PRONOM registry entries for .ots.