Old English spelling wasn't entirely uniform. I don't know how to judge the precise degree to which it was non-uniform. Old English words could have more than one spelling, and dictionary entries certainly make use of normalization: the most common normalizations are the use of symbols like ċ, ġ to represent palatal consonants and ā, ō, etc. to represent long vowels, the exclusive use of w instead of ƿ, and the use of only one of þ or ð.
The letters þ and ð both represented the same sound(s) in Old English (typically analyzed as a single phoneme /θ/ with two conditioned allophones, voiceless [θ] and voiced [ð]), and there are examples of the same word being spelled in some places with þ and in others with ð. For example, Beowulf uses the spellings æþeling and æðeling.
"Old English" comprises at least four main dialect groups that Wikipedia gives as "Mercian, Northumbrian, Kentish, and West Saxon". There were differences in pronunciation between these dialects that were made manifest in various spelling differences.
"Essentials of Old English" (University of Glasgow) mentions the existence of variation between the letters "a" and "o" before nasal consonants.