Olympic was the first of the new class of ocean liners from the White Star Line intended to be the most luxurious on the market. This is in the days before air travel, so the only way to cross the Atlantic was by ocean liner (not to be confused with a cruise ship). Being first of the class, like most first-time products, meant that the later sisters overall had the same shape and basic design, but also had slight improvements on the Olympic. Because the Titanic sank on its maiden voyage, there are very few photos of the ship, particularly the inside. Most of the shots used to show the interior of Titanic are actually those of Olympic because in most respects they were identical. They had the exact same base plan and were largely built side-by-side. It's not uncommon for exterior shots of Olympic to be mistaken for Titanic since they had very similar exteriors and identical paint schemes. There are differences, though, that are easy to spot.
The biggest visible difference between the Olympic and Titanic was on the forward Promenade Deck (originally labeled as A-deck, the first deck below the top level or Boat Deck). The Olympic's promenade was open the entire length. During its initial runs, there was apparently some complaint about spray from the wind, so Titanic had the forward part of the promenade enclosed. The later sister ship, Britannic, also had the forward promenade enclosed, but Olympic never did despite having several refits over the course of its career, many of which added features the Titanic had. This is probably the most frequently used feature to distinguish the two ships, though it's not a perfect difference. Titanic did not have that modification made until the very end of construction, so early shots have the same fully-opened promenade as Olympic, which often results in some Titanic enthusiasts believing they are looking at Olympic when, in fact, it is Titanic in latter stages of construction.
|Comparison picture from "The Great Titanic Switch" highlighting some of the differences between Olympic and Titanic
There is another way to distinguish the Olympic from Titanic in the early photos, but it's very small. On the bridge level, there were two "wing" cabins on either end. In the original configuration, the outer walls of these cabins on the Olympic were flush with the edge of the ship. On the Titanic, however, these cabins protruded 2 feet over the edge of the ship. After the loss of Titanic, several changes were made to Olympic, one of which was altering the wing cabins to protrude outward (among other changes; the bridge of the Titanic was quite different than the original configuration of the Olympic and was a major improvement).
There are several other small differences between Olympic and Titanic, all of which have been documented and studied due to the conspiracy theory that the ship that sank was actually the Olympic redressed as Titanic. While the idea is tantalizing to say the least, the idea that the ships would be switched (allegedly because the Olympic had received major damage in a collision with the HMS Hawke and was sunk on purpose to get insurance money), it's pretty much impossible not only because the immense amount of money required to change them, but the very obvious problem that Olympic's parts were stamped with the hull number 400 and Titanic's were stamped with 401. Pieces of Titanic recovered from the wreck have had 401 on them. Oh well!
|Britannic painted in hospital ship colors. The huge lifeboat arms can be seen here as well, a defining characteristic of the ship. Like Titanic, Britannic had a partially enclosed upper promenade.
|Cool picture of Britannic under construction with the Olympic's funnels visible in the background, from Kevin Scott Bolinger's post
Olympic served a long and eventful career, finally being retired in 1935 and scrapped about a year later. It served as a troopship during World War I and actually rammed a German U-boat in that capacity and, of course, had a long career as an ocean liner between New York and Southampton. It is likely that if Titanic hadn't sunk, it would have also been used as a troopship in the war or some other military purpose.
|This painting and the picture below show Olympic in "dazzle paint" during its time as a troopship. The dazzle paint was supposed to act as a camouflage to protect against submarine attacks