Interestingly, a one way ticket will not always get you the SSSS. In December I was in Albany during a snow storm. I was scheduled out of EWR, but there was no way I was getting down the highway to make my flight which was cancelled anyway. So into the ALB airport and to the NW counter. FLight to MSP was cancelled already, so they booked me on the flight the next morning. AA still had a flight going out that would get me back to MSP. NW tried to Rule 240 me over to AA, but couldn't figure out how to do it b/c my original airport was EWR. Realizing that if I wanted to make the flight out I just needed to buy a one-way on AA and sort out the rest later.

Bought the ticket on AA, one way, paid with my amex, had a boarding pass handed to me without the SSSS. I admit I was expecting the SSSS, but it wasn't there. I don't know why it didn't show up. I hadn't flown AA in over 15 years, and had no other association with them. But obviously there are other triggers in the system besides the one-way that cause the SSSS. It may be that the one-way gives you enough "points" that a single other factor will cause you to be SSSS. But who knows what it takes.