The closest living language to Old English other than English and Scots is Frisian. After that it'd be Dutch and German. The Nordic languages are further removed still, which is where Icelandic fits in, although Old Norse had shaped Old English a fair bit.
Frisian is sometimes even rather like New English, which can be seen in the saying, "Butter bread and green cheese is good English and good Fries." It's written in Frisian as, "Bûter, brea, en griene tsiis is goed Ingelsk en goed Frysk." and is said almost the same in both tongues. (Works well when spoken in a Lancashire accent.)
A blend of speaking and reading Old English to another would help, as some words might spell kinda the same, whereas other words sound kinda the same.
Take English alike, and German gleich. These words are cognate and both mean the same thing, but it might not seem so at first look. One one hand, the spelling is fairly different, and the first syllable does not look related at all, but the second bits sound rather alike. You may never know it, but the 'a' at the beginning of the English word and 'g' at the start of the German word both have the same roots and mean the same thing. In Old English, the word was 'gelic', where ge- means 'being/doing'. (Also found in words like 'aright', as in 'to set aright', rather than 'something that is right'.) We say it now as 'ah-laik'; in later OE it was said as 'yeah-litch'; and in early Migration-age OE it would've been said as 'geh-litch'. So now you can probably see the connection between English alike and German gleich.