Oom means “uncle.” Like in English, the Dutch word oom is used for the brother of a parent, or the husband of the sister of a parent. There is no separate word for paternal or maternal uncle.
You may encounter references to ooms in guardianship records, estate inventories, as specification of how witnesses are related to the main parties, and in many other types of records. Records do not usually differentiate between full and half uncles, or between uncles-by-blood or uncles-by-marriage. Sometimes you may see someone referred to as oom van halven bedde [uncle of a half bed, meaning half-uncle] or as bloedoom [blood uncle, uncle-by-blood].