You can use an IFrame to embed external content using the standard Oak content editor which can be found in places such as the custom homepage applet, basic and news pages. In the editor you can use the 'source code' option which shows you the HTML code that makes up the page. You can enter this mode by selecting 'tools->source code' from the editor menu.
For an iFrame to work, the content itself needs to come from a secure website that begins with "https:", and the site address must be added to the 'external sources' list which you can find in your site settings.
To use an iframe, you simply insert the following piece of code into your content (while in source code mode, as above):
but replace the text saying 'URL' with the code you wish to embed from the external source you wish to pull content in from.
Wherever possible, we'd recommend that you use the official 'share' function many sites make available for this purpose (e.g. YouTube). Please be aware that some sites deliberately do not permit their content to be shared using an iFrame and in this instance you will not see the content show in your page.