Diff. between 己 and 自身?

Question:

like... one's おのれ and the other's 自身　but they mean the same thing? I'm guessing they're used differently though... so any guidance would be great. Thanks.

Answer:

Daisuke is correct.There is a difference between おのれ and 自身

おのれ is a pronoun (myself, yourself, himself or herself). it can be use as third person pronoun.
For example:
先生は己に対して厳しい。（The teacher is strict with himself)
己に勝つ（control/master oneself('s desires))
己を省みる（reflect upon oneself('s actions))
己の失敗で気がとがめる（feel guilty about one's mistake)

おのれ has a second meaning for You. This is second usage can only be used if the person you are addressing has an inferior status compared to you. (Japanese is a hierarchal language)
For example: 己の名を言え（Give your name!)

自身 means personally. For example,
文部大臣自身が来られた。（the Minister for Education came personally)
私自身はこの絵のほうが好きです。（Personally, I prefer this picture.)
君自身の意見はどうなんだ？（What is your personal opinion?)
僕自身は傷つけた。(I personally hurt/injured myself)