Old English (OE) was written with the same writing system as we use for Present-day English (PDE): the Roman (or Latin) alphabet. However, the way in which the sounds of OE are represented in written forms of the language varies in a number of ways from PDE, both in terms of the range of letters/graphemes it uses, and in terms of the sounds those graphemes represent.
There are some symbols/letters in Old English that are no longer present in English today, and some PDE letters are not found in OE.