[77'911]

Replace the sensor with a factory unit. O2 sensors start getting marginal around 60k, 90k they are usually toast depending on the service/driving history on the car. At 119k, its time.

 

[77'911]

He said he replaced them with OEM O2 sensors.

Yep, OEM doesn't cut it. The quality is not there.

 

[77'911]

What brand do you recommend?

I'd stick with factory subaru, especially for electronic parts(IE sensors, cooling system, etc..).

 

[77'911]

OEM, original equipment manufacturer, is factury Subaru.

No. it is not. Without going into a dissertation, the companies now tier quality with separate entities("aftermarket division") that are under the same umbrella. Caveat emptor. It's not like it was 30 years ago.