Document

What is a .otp file?

A .otp file is an OpenDocument Presentation (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 .otp over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .otp 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 Presentation, OpenDocument Presentation, OpenDocument Presentation, so software from that family is the most likely match.

How to convert a .otp 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 .otp file travels over the web, servers label it with this MIME type:

application/vnd.oasis.opendocument.presentation

What's a MIME type?

The technical record

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