WOW Airlines has San Francisco to Frankfurt for $100 one way and $158 including one carryon ($40) and self selected cheap seats ($18).
There are 23 dates with $100 fares from Sep 11-Dec 12 at time of this post. This is a great deal for one way travel to Europe.
$291 is the lowest base fare for WOW Frankfurt to SFO one way before bag and seats, so this is not a great deal for round trip travel on WOW. There are some dates when SFO-Frankfurt is $320 round trip, before bag and seat fees and about $440 with self-selected seats and one carryon bag. The current WOW sale has different prices when searching one way or round trip, which is a situation I do not recall seeing before with WOW ticket prices.
Norwegian has $150 one way fares Stockholm ARN to Oakland and $148 Oslo to Oakland. This is the way I would play this fare if looking for a round trip by traveling from Frankfurt to another city for low airfare to Scandinavia. Might be even cheaper to fly Frankfurt to London $30 to $40, and pick up a sub-$200 one way Norwegian flight London Gatwick to Oakland.
WOW $100 SFO-FRA dates
October: 2, 3, 10, 21, 24, 26
November: 2, 7, 9, 11, 28, 30
December: 2, 5, 7, 12
WOW Airlines San Francisco – Keflavik, Iceland KEF – Frankfurt FRA