We got our first good look an an Olympic class ship in "All Good Things", when the USS Pasteur was commanded by Captain Beverly Picard. At that time I took it to be a completely new future design, but I'm assured by several sources that Olympics are already in service in TNG times. I've made them fairly new designs to explain why they are apparently still front-line ships in the 2390+ timeline of "All Good Things".

The idea that the Olympic picked up survivors after Wolf 359 is to explain the discrepancy between "Best of Both Worlds" and "Emissary". The former had the E-D pass through the battle area some hours after the battle with Data reporting no sign of survivors, while the latter showed Commander Sisko and many others escaping in pods or shuttlecraft of some sort. If a regular starship picked up the survivors you'd expect it to have headed off after the Borg; if an unarmed Olympic did the job it would head for the nearest Starbase, leaving the area deserted when the E-D passed by.