Hi Sok Fung,
MTM gowns means the designer will sit down with you to discuss what design, style, material you like for yr gown. From there, they'll sketch out the gown based on your requirement before proceeding to make our gown. Prior to that, they might also let u try on some ready made gowns (can be MTM gowns from previous brides or New OTR gowns), to let you feel what kind of style you want..eg; dreamy, grand, etc.
For OTR, there's new OTR or worn OTR (previous MTM gown /OTR gowns wore by brides), it also depends on your BS..cos' some designer BS don't have new OTR.
When you step in the BS, you might want to look ard at the gowns, you can also request to try on some gowns to see if you like the designer style, etc. Also, look at their pkg and talk to the designer..etc
My 2cents worth.