"the word of truth" refers to all of Scripture in general (John 17:17), and the gospel message in particular (Eph. 1:13; Col. 1:5).
The thought is that the minister of the gospel is to present the truth rightly, not abridging it, not handling it as a charlatan (see on 2 Corinthians 2:17), not making it a matter of wordy strife verse 14), but treating it honestly and fully, in a straightforward manner. (copied directly from Vincent's Word Study)
Is it your claim that you are ONLY to rightly divide the OT?
For me, I study ALL of the scripture because it is His Word. The Old Testament SCREAMS OUT our need for a Savior... And declares God's plan for mankind. (Even to include salvation by grace - not works - to put a fine point on the matter.)