Undercut Metal Door Jambs
Use an angle grinder.  I recommend practicing though I find angle grinders one of the easiest tools to control (no crazy binding like drills).  Really you should buy one they are $15 USD at harbor freight.  If you are going to use it more than once splurge and get the $30 one.  Any metal cutting disc will cut that quick.  Just get a thin kerf (3/32&quot;) blade, wear eye and ear protection and keep any plastic out of the way of the hot metal particles.  Won't hurt to have an extinguisher near by.
I've used oscillating tools on metal - the blades dull really quick.  A reciprocating saw can work but you have limited clearance.  Even then the reciprocating blades can dull quick quickly on metal as well.  I can't imagine using a jamb saw on that.