A file with an .ai extension is an Adobe Illustrator Artwork file that contains vector graphics on a single page. It uses points to create paths for displaying the image data, thus making it safe from losing image quality if it is enlarged. AI format finds its major usage for logos and print media. AI files can be opened with Adobe Illustrator, Adobe Acrobat DC, PaintShop Pro, and CorelDraw Graphics tools.
Files with OTT extension represent template documents generated by applications in compliance with the OASIS' OpenDocument standard format. These are created with word processor applications such as free OpenOffice Writer and can hold settings that can be used to generate new documents from these template files. These settings include page margins, borders, headers, footers, and other page settings. Such templates are used in official documents such as company letterheads and standardized forms.
You can also convert AI into many other file formats. Please see the complete list below.