I WAS THERE majinD

(ð is lowercase Ð, btw)

It's also not a rule in Old English, only in Icelandic. In OE, þ and ð both represented either "th" sound, and scribes used them mostly at random. There are manuscripts where all four spellings of the word oððe ("or"), oðþe, oþþe, oððe, and oþðe, get used in the same paragraph. It's pretty funny.

Some corrections: Þ/ð were completely interchangeable in OE, both could be either th sound. They only represent different sounds in Icelandic. œ was /ø/, the Norwegian ø/German ö vowel. It unrounded to /e(:)/ later though. And ƿ is just /w/, it comes from the Germanic rune used for that sound.