The main function of the Orion, in my opinion, is so you don't have to field all Thunderbolts. The venerable T-Bolt is one of the most effective units in the game in terms of firepower and armor for the tonnage, carrying better protection than some Assault 'Mechs at least 20 tons heavier. The LRM-15 rack gives it respectable ranged firepower, although not quite enough to compete with dedicated fire support designs. Up close and personal, it can deliver a LL, 3xML, and 2xMG (27 potential damage) while walking, and still not build enough heat to suffer a movement penalty the next turn, or throw in the SRM-2 rack for another 2 heat and a movement penalty. Loaded with Infernos, that can seriously degrade the performance of most other intro-tech designs, and the MGs give it anti-infantry capability. In short, it can do everything reasonably well except scout. While it carries a distressing amount of ammo, that's fairly well padded by other internal equipment, so MOST players don't see explosion issues with any regularity (a few obviously have different luck). There's a jump-capable version at the expense of other options, for those who want to take advantage of that.
The Orion would be an impressive infighter/brawler if not for the severe shortage of heatsinks, with an LRM-15 rack (as with the T-bolt) to reach out and touch something before it gets there. The AC/10 exceeds the damage potential of the T-Bolt's LL, but is ammo dependent, so you need to be slightly more cautious about its use. The short-range armament comes down to 2xML and different numbers of SRM/4 racks mainly at the expense of armor, depending on the version. Like the T-bolt, it has a significant number of ammo slots, which in this case are slightly less well padded. Depending on version, it may carry significantly less or slightly more armor than the T-Bolt. Given the choice, I'd generally take a T-Bolt over an Orion, but only by a fairly small margin. I'd put money on the T-bolt in a duel between them, even though the Orion can do quite a bit more physical damage.