The pharmacological profile of Tramadol is far more complex than that of plain o-desmethyl.
Tramadol itself has a strong affinity for SERT but weak affinity for NET and opioid receptors. Both O-desmethyl isomers have a negligible (none?) affinity for SERT and a high affinity for Mu and, in the case of M1 (forgot which isomer this one is), a high affinity for NET.
What this means is that when you take tramadol, you experience 3 distinct and very synergistic effects. When you take o-Desmethyl, you experience one less of these, and thus the synergy is very different.
For all intents and purposes, they are two completely different drugs.