Why would Jans fare any better? They are both equal against knights, in fact Oromos are better due to Drill 2 opening access to the Formation promotion to counter mounted units.
For general counter attacks, Oromo first strikes make them more likely to survive with no damage, the Drill line reduces collateral damage and they are naturally immune to first strikes, and the the Drill promos don't obsolete with the unit!
I would argue that Oromos are quite a lot better than Jans, though Jans are definately a very good UU.
I do agree with the "muskets are underrated" sentiment in this thread, draftable, hard to counter and effective general purpose units that come on the way to Steel are hard to turn down!