How are reading and writing connected to developing good oral communication skills?

Human communication skills can be broadly categorized in the following way: reading, writing, speaking, listening, and body language. These forms of communication are often combined in various ways to deliver ideas, emotions, and thoughts.
 
There are some properties of communication that span all of the different forms of communication, and as such, practice in one communication form can help improve your communication skills in another form. Strong reading and writing skills, for example, can help develop your oral communication skills. Below are listed a few qualities of good writing that are also useful for good oral communication.
 
Organization
Clear organization is helpful for communication in all forms. Organizing your thoughts makes it easier for your message to be received by a reader or a listener. Disorganized communication can be hard to follow and your audience may not understand the message you are trying to deliver. Organization in writing is a pre-planned mindset - thinking about how you will organize your essay or article beforehand is critical to delivering your message in a comprehensible way. Similarly, in oral communication, planning ahead is a good idea so that you deliver ideas in an order that makes sense for the content.
 
Conciseness
Conciseness is limiting your message to only what is needed and cutting out any filler. If your message continues on too long, your audience may become disinterested and lose the ability to follow along with your ideas. In writing, you can edit out any extra sentences that are superfluous. Similarly, as you plan oral communications, you can look ahead to remove any parts of the speech that are unnecessary. 
 
Clarity
All forms of communication require clarity in order to be effective. Clear communication is direct and contains no contradictions. Ideas stated should be correct to the best of your knowledge - do not provide information that is incorrect. Additionally, in order to deliver a message that is clear - whether written or oral - that message should be complete. Give your audience the full set of information.
 
These are just a few ways that communication skills are transferable from one medium to another. It is also worth mentioning that practice in any one form of communication can help you build your basis of communication skills in general, and that foundation will serve you as you move into another form of communication.