[canjobear]

Knowing German would mostly be helpful for understanding the grammar of Old English. The three genders and four cases, participles prefixed with ge-, verbs like sindon (=sind). There are tons of cognates with German (like þurh = durch) but they're hard to recognize immediately unless you know the kinds of sound changes that are common.

[LAC-Tech]

I guess the concepts and some of the vocab are important (though I feel compelled to point out that þurh is cognate with through as well).

But Old English inflecting nouns, rather than relying on indefinite and definite articles, gives the language a very different quality to German. Also stuff like negative concord.