Recently a leader in the Alberta OER community, Michael McNally, suggested that it is difficult or impossible to only use open source software (OSS) when creating open educational resources (OER). I agree with his point that using only OSS doesn't make OER more "pure", but perhaps it is still an interesting challenge.
Here are some of my suggestions, please comment if anything is missing. And I do understand the hypocrisy of posting this on a Google-hosted blog.
Text is still often the primary medium for OER, and there are a number of great open-source text-authoring tools. LibreOffice is a great office suite, and it is similar to traditional office suites so there shouldn't be much of a learning curve.
If you prefer collaborative writing, perhaps check out Nextcloud. You'll need to host it somewhere, if you are in Alberta consider Cybera's Rapid Access Cloud which uses OpenStack.
Diagrams and Graphics
Inkscape is a great vector editing and layout program. For image editing and creation, check out GIMP, Krita, or MyPaint.
One of the best simple audio recording and editing programs is also open source, Audacity. There are others, of course, but it should do everything you need.
My favorite open-source video editing program is Open Shot, but you may also want to check out Shotcut.
Hosting video is another issue, though. You can host videos in a learning management system such as Moodle, or check out alternatives such as MediaGoblin, Kaltura, or ClipBucket.
Linux has gotten much easier to install and use if you'd like to replace Windows or MacOS. My current favorite distribution is Peppermint.
As previously mentioned, Albertans can avoid the big five cloud providers by running servers on RAC, but your institution may have self-hosted instances of Pressbooks or similar open-source hosting platforms.
Hopefully those cover anything you may need to use when creating OER with OSS. In some cases these tools are preferable to commercial products.
Of course if you are philosophically opposed to proprietary software then you are probably already familiar with most of these.
As always, please comment if you have any other suggestions.