You can choose which file format (such as .pptx or .odp) you want to be the default when you're saving your presentations.
Note: Not all features are supported in OpenDocument format. For more info, see Use PowerPoint to open or save a presentation in the OpenDocument Presentation (.odp) format.
Switch your default file format
Select File > Options.
Under Save presentations, in the Save files in this format list, choose the file format you want to use by default.