Kana spelling is common in everyday writing, especially when the age distinction is not important.
Kanji spelling for uncle, especially when older than one's parent. Also used generically.
叔母 means aunt (parent's sister), while 叔父 means uncle (parent's brother).
The reading おじ is native Japanese. The kanji 叔父 and 伯父 are Chinese borrowings that distinguish younger and older uncles, respectively.