When the Angles, Jutes, and Saxons arrived in England around fall of Rome in the early 400s, the "official" language was Latin (as Britain had been a Roman colony since 43 AD) whereas locals spoke various Celtic dialects. The newcomers brought with them Scandinavian tongues and for the next 500 years or so Old English developed with minor changes (with sparse inclusions from Latin and Celtic influences). That all changed in 1066 with the Norman Invasion where Old Northern French became the new official language. (French also began as a Nordic language, but over time only a few hundred or so words remained, with the rest being mostly Latin-based.) As far as Modern English is concerned, while only ~20% is based on Old Norse, those words form more than 80% of what is most commonly used on a day-to-day basis.
What's up with the phrasing? Old English isn't a language that developed from Old Norse.