I just don't see how OB can do more damage than SS in an unholy PVP build replacing SS. Even with 20% armor penetration lets take a look at these 2 abilities.
Glyphed OB does 100% weapon dmg plus 467.2 plus 12.5% dmg increase per disease
Scourge Strike does 40% weapon damage plus 135 plus 10% dmg increase per disease
Now some things to take into account in PVP. SS glyphed has a 25% chance to stack 3 of your diseases per strike, as well as it drops a 50% aoe snare everytime you use it, where as OB does not.
In addition SS ignores 100% of armor as it is a spell based strike.
That already in 1 way, just utility wise, SS seems better for an unholy PVP Spec.
Now lets look at the talents that boost these 2 strikes in a typical unholy spec.
Obliterate -
3 points in blood if you want for 9% crit from 3/3 Subversion
3/3 Anhilation in frost would be mandatory to using OB
If you go 20 points into frost you can get Chill of the Grave for 5 more RP per OB strike
For 2 points in Dirge in unholy you get an additional 5 RP per OB strike
Now none of these talents, except for Subversion, which would require you to go 3/17/51, boost the damage of your OB leaving it pretty much at stock damage as posted above with glyph.
So now lets look at the talents that boost SS.
Scourge Strike -
10% increased damage from 5/5 Black Ice in frost
2/2 Vicious Strikes giving 6% crit and 30% dmg bonus to critical strikes
3/3 Outbreak giving you 20% extra damage
5/5 Impurity giving your SS 20% benefit off your AP
2/2 Dirge giving you 5RP more per SS
2/2 Desecration giving you SS an AOE snare 50% speed for 12 sec
5/5 Rage of Rivendare giving SS 10% more dmg agaisnt diseased enemy
3/3 Ebon Plaguebringer increased SS dmg by 13%
6 seperate talents that all directly boost the damage of SS versus just 1 for OB
So with all said and done, and in addition SS ignoring all armor thus allowing you to gem strictly for damage, I really don't see how OB > SS in an unholy PVP build.