To be brutally honest, I am saying that Prissy x Oswin happens 0% of the time. Let's look at other people who Prissy can support.
Prissy x Erk - Starts at the same time as Oswin but Erk has a small move advantage over Oswin. He also has a 15 +3 support with her which is usually better than 0 +2 unless you're bad at math. Erk gets C Prissy in 22 turns and then a B in an additional 27 turns. Oswin gets C Prissy in 41 turns, 8 turns before Erk nails the B. Let's check out the benefits:
ThunderxWind (Erk) is +1 Att, +1 Def, +5 Hit, +10 Crit and +10 Crit Evade. Considering that level 14 Erk!Thunder has 10 Crit and 18 Mt during Kinship's Bond, this support ups his Crit to 20 and gives him 19 Att vs. pathetic Res counts of like 4. He's doing about 15 damage a pop and doubling while 17 Oswin!Steel Lance has 26 Att vs. significantly higher Def numbers and fails to double, unlike Erk. The AnimaxWind (Oswin) support of +1 Att, +2 Hit, +2 Crit and +5 Crit Evade is just... meh when compared to Erk's.
Raven x Prissy - No niceboat jokes, please. This support is also miles faster than Prissy x Oswin. Even though IcexWind is terrible, Prissy gains no better bonuses from Oswin while Raven gets a working support himself. Granted, Raven's gonna want B Prissy because A Lucius is a better support and he has no other options for supports.
Prissy x Guy - Much better support for Prissy than Oswin because she offers Guy something that he wants (full Crit) while Oswin doesn't get what he wants from her (Hit). It's also 0 +2 and is just as fast (or slow if you want to put it that way) as Oswin x Prissy even though Guy has a slight move advantage on Oswin.
Sain x Prissy - Same speed as Oswin x Prissy but WindxWind benefits Sain more than WindxAnima benefits Oswin (Sain gets 1+ Att and +5 Crit for every support level). Sain also destroys Oswin in mobility so waiting about 2 chapters to start supporting Prissy will start paying itself off once Sain gets that first Att and Crit boost.
Heath x Prissy - If you're waiting this long for Prissy to support someone, you're doing it wrong. However, I've already mentioned the perks of WindxThunder (see Erk) and Heath likes the full Crit from this support. That being said, it's more likely to see Nancy Pelosi snort cocaine off Newt Gingrich's cock than it is to witness Heath profess his love towards the red haired girl, the little cunt that he is.
Point is that Oswin is Prissy's 5th best support out of 6. It's too bad that only numbers 1 and 2 will enjoy the benefits of this support. As for Oswin's other supports, the only way they'd be ever be considered "h4x" is if the movie Idiocracy came true (people water plants with energy drinks). Serra x Oswin is garbage, Hector would rather B Matt or Lyn (both are faster than Oswin's support and give Hector full Crit), Dorcas has a very good chance of not getting played lategame and even if he did, he'd want B Oswin at best (A Bartre for him and I honestly don't think I'm pushing the envelope by saying that Dorcas x Geitz is much better than Dorcas x Oswin) and Matthew's off stealing shit and won't be guaranteed to be in range of Oswin to give the bonuses, never mind stand beside him and get the actual support.
Claiming that you're overestimating how easy it is to get supports is like pointing out that a kid in a wheelchair can't run a marathon. It's obvious that both statements are true (his inability to walk and your bullshitting of supports) but neither of you really give a fuck about what anyone says. Portsman, Colonel and I have all pointed out numerous times that you constantly inflate levels and supports but you somehow don't seem to listen.
PS: In case you couldn't tell, I hate Heath.
EDIT: Thanks Narga.