There is only one file type that uses the .OTC extension:
OpenDocument Chart Template
Chart template created by programs that implement OASIS's OpenDocument standard; stores the default setting and layout information for a chart and may specify the dimensions, plot area, and axis information; used to create multiple .ODC files with the same look and feel.
ODC files are XML-based, similarly to other formats in the OpenDocument standard. They are stored as compressed Zip archives, which may include multiple XML files and other assets.
NOTE: OpenOffice, which was previously StarOffice, is one of the most widely-known programs that uses OTC files.
Updated: April 13, 2014