To unlock this special model some events must be done. After the completion of the mission of Area-M make sure to not leave the area after sending your mission report. Saving is crucial afterwards so you will not die from Omega Zero the final Boss from Mega Man Zero 3. Go to the other side of Transerver where it reads Area-N. Go to the second floor of the stage. Keep going until you see re-appearing metal floor steps. Proceed further until you see a floor of spikes. More of those metal floor steps are there. It will be faster to reach those steps with Model HX. After that there is a door. Once there you will fight Omega Zero. You should have Model ZX and at least two or three subtanks to defeat Omega Zero. After he is defeated go back to where you started to get to Area-N. This time go to the last floor until you see two doors the require the Mega Man Zero 3 or 4 Game Boy Advance cartridges. Beyond those two doors is a room where a floating mysterious rock is hovering. Get it then leave the area. The model can only be used after completing the game and playing your cleared saved game file. When doing this playing your cleared file talk to Flueve. Give him the mysterious rock and he will confirm its a BioMetal. You are now in possession of BioMetal OX.