Hey, Simon! You can install some of the third-party WYSIWYG text editors from the store to accomplish this. However, currently the best ones are no longer supported so I'd steer clear of what could be a frustrating purchase. If Oxwall users make it known that there's a current gap for a plugin like this, perhaps a more active developer will take it on.
You can look into outside open-source options such as TinyMCE. For reference, this is the text editor used for WordPress. I'm currently gauging this option myself but it's low-priority for me, so I won't have any updates soon.
Hope that helps start you on your trek. Apologies that I couldn't be more helpful.