There are several steps that you need to take in order to strengthen your vocabulary. One of them, rather obviously, is to learn new words. But not just any new words....
That's the problem with "Learn a Word a Day" programs. Do you really need to be comfortable with the word "obtuse"? Or "obfuscate"?Or "oxymoron"? (Just to mention several not-so-common o-words.) Well, yes, you do.
Or maybe not.
Whether or not you should use a word largely depends on your audience. If they're not comfortable with a word, you probably shouldn't use it.
On the other hand, if your audience is comfortable with a word, it helps if you not only know the word, but can use it effectively. So if you're speaking to a specialized audience, you might need to learn a specialized vocabulary.
The following exercises introduce you to some specialized vocabularies that, as a speaker, you might need to be familiar with.