In both the OT and the NT, the Word of God (debar Jahweh or λόγος Θεού) has many meanings.
In the OT, debar Jahweh often involves the actual, physical voicing of God's message. The Word of God is not something that is written down or studied, but something that is spoken, heard and responded to. After all, we are talking about a largely oral culture, especially in the OT's case.
In prophetic settings, for example, the person is inspired by the Spirit of God and then either uses their mouth/tongue to speak the word of God (e.g. 2 Sam 23, Num 24), or they hear a voice that is the word of God (e.g. 1 Sam 3, Jeremiah 1:11, Amos 8:2). In these cases, dabar is something that is very dynamic, intimate, personal, i.e. founded on the unique relationship of God's Spirit to a particular person. This person's word conveys the Word of God, which is experienced as truth by the hearer.
As Mediterranean culture became more literary, the understanding of the "word" changed. Thus, in Hellenistic times, there are sources that speak of the actual corpus of inspired writings as the "word". For example, the NT refers to the Septuagint as as a human word (λόγοι Ήσαΐου, Jn 13.38, Lk 3.4) and also the word of God (λόγος Θεού, Mk 7.13; Jn 10.35; 2 Pet 3.5-7). Even in the NT, though, there is still a heavy emphasis on how the Word of God must be vocally proclaimed, heard and responded to. Proclaiming the Word of God includes reading aloud the writings of the Septuagint, but, even more than that, preaching the Gospel of Christ.
As others mentioned, St. John's Gospel goes to great lengths to explain how Jesus himself -- his very person -- is THE Word of God, something which St. Ignatius picks up on in Magnesians 8.2. From then on out, most Christians regularly speak of Jesus as the λόγος. By the time of the great fourth century fathers, it was more common to call Jesus the λόγος than it was to call him Jesus. Just think of the title of St. Athanasius' famous tome. It's On the Incarnation of the Word -- not On the Incarnation of Jesus.