It's a 2009 commissioned stealth frigate, with every navigational aid you can think of (AIS, a range of radars, visible light and FLIR cameras) on the bridge. They also had repeated warnings over the radio.

I'm not sure if the radio transmission from https://www.vg.no/spesial/2018/helge-ingstad-ulykken/ (linked already in various comments) is accurate, but if so, there were no warnings over the radio. Instead, they were directed to change heading but not informed why. I don't speak Norsk, but it seems like the first time "collision course" is mentioned is at 04:01:08, ten seconds before impact.

The first call to the warship was 03:10, some 1:20 before the collision. Followed by two warnings they are on collision course.

Let's see it from a different perspective too. The oil tanker captain was able to identify an unknown vessel (with disabled transponder) as warship, guess its type and infer its name, determine the collision is imminent and warn them multiple times. This suggests absolutely inept crew on the frigate's bridge.