Not very. There is an idealized normalized spelling used in learners' materials and reference resources like dictionaries, and nowhere else. Heck, people can't even agree on which normalized spelling to use: some prefer the conservative Early West Saxon forms (like hīeran), since they better illustrate the history of the word, while others prefer Late West Saxon forms (hȳran), since Late West Saxon is much better attested.
But both of them exhibit much variation in spelling. The letters þ (thorn) and ð (eth) were completely interchangeable, and many manuscripts used both of them freely. They did tend to use þ at the start of a word and ð at the end, but this was only a tendency, and in the middle of a word, all bets were off. The word oþþe could also be spelled oðþe, oþðe and oððe, and all of these spellings were reasonably common.
The letters a and o were largely interchangeable before the nasal consonants n and m. The word mann can also be encountered as man, monn, or mon. The infinitive form of a verb was usually spelled with -an and the past tense plural form was usually spelled with -on (thus "to love" is lufian and "[they] loved" is lufodon, but less careful writers may still interchange these endings.
For other examples of variation, the word cyning might be encountered in the wild as kyning, kining, cyningc, cynincg, etc. (For some reason, the letter k is much more common in this word than in most other words.)
However, despite all this, Old English spelling is not especially difficult to deal with. If you already know the word cyning, then it'll be pretty obvious what a kiningc is, especially when you see the word in context. There are still underlying rules that explain most of the variant spellings that might occur, and finding a word's normalized form is just a matter of applying those rules in reverse.