I find myself running in the Osiris B even when I have the money for a Heavy/Assault.
First, it's FUN! Running at 164KPH in a light mech really makes for "seat of the pants" encounters. I know if I stop, I'm dead. On that note, I find using the JJ's usually means death since you stop all forward momentum when you land. Everything about this mech is based on speed, so even hitting a small rock (or a tiny drop off that causes speed to reset) means I'm toast.
I find my roll to be that of a harasser. I get in behind the enemy and play the opportunist. Also, it's ability to cause havoc amongst a group of mechs is underestimated. Often I'll find friendly heavy/assault mechs that are advancing on an enemy and blaze past him to distract the enemy while he comes into range. You can really disrupt a group of snipers/LRM boats with the Osiris. You don't kill them fast, but you can occupy them for a good while.
It doesn't have AMS, but if you're moving fast (and smart about direction) missiles can't hit you. My largest threat is a 3rd mech firing when my transversal velocity is at a minimum. Or, again, anything that slows my speed. If my rear torso takes damage and slows my engine, I'm also easy prey.
On the note of the torso, I seem to find a disproportionate amount of damage hits my rear. Even in 1 on 1 circle of deaths, my rear torso seems to take the brunt of the damage. I'm not sure if this is a modeling issue or not, but perhaps something to be mindful of.
Thanks for this little gem of a mech!
- Bogeyman