Which Bike ?

Its not a matter of better bike.

Find a tri bike that fits, they have steeper seat tubes and shorter top tubes. The OCR is not a tri bike, it is a comfort bike. If you like Giant, look at the TCR for tri use. Felt is another good option, as is Cervelo.

Respectfully, I have to disagree with the last opinion. Mountain bikes have a fairly relaxed geometry (seat tube angle). A triathalon specific bike is a pretty violent shift and I expect you to perhaps find the change to be too much to adjust to, at least for comfort's sake. A good all around road bike offers more flexibility of usage. There are a ton of variables that you did not mention (price range, what type of MTB you usually do, etc.) For a really solid road bike I would look to spend at least $900 to $1200 with a minimum componentry level of Shimano 105. Ultegra 10 is my current favorite, but 105 is a very solid group, especially if you don't put in a ton of miles. There is a ton of value in this price range so shop around a bit and look for 2005 closeouts for best value. Slap on a set of clip on TT bars and you will be just fine for recreational level TT. (I have a Cervelo Prodigy that I raced in our Corporate Challenge TT last year, besting our two other guys on far more expensive dedicated TT bikes of the same make). Depending upon size and the condition of the roads in your area, you might be able to get away with aluminum or an aluminum/carbon mix. Steel rides very nice, but is a little heavier. Aluminum is stiffer and less comfortable, and the frames tend to fatigue and have limited life. Carbon is my current favorite and titanium has great advantages too (great fatigue life, doesn't corrode). If you are interested in mail order, Colorado Cyclist is selling the Litespeed Firenze or their house brand Douglas Precision (full ti) or Precision Plus (ti with carbon seat stays) for about $1999 with Ultegra 10. Performance has the 2005 Scattante CFR LE carbon for $2299 with full Dura Ace 10. I have this bike under the name EPX 303 and have been very impressed with it.

Since you are new to road, your best bet is likely to avoid mail order and ask around to find a good shop. It is worth it as they will let you test ride various bikes and can help you a lot with fit. If you find that a stem is a little short/long they are usually really good about swap outs. I have been riding road for years so I am comfortable buying M.O. or Ebay, but I would recommend against this if your experience with road bike fit is limited.

Best of luck.