The first clue describes a word, which, when O is added as a prefix changes to a word meaning the second clue. It is like: first clue + O (as a prefix) = second clue.
-1.O with a preposition meaning position makes a delicious cereal. 3 letters
-2.Variety with O becomes a colour or a fruit. 6
-3.A writing tool with O is never shut. 4
-4.O with you and I leaves us in debt. 3
-5.The second musical note with O makes a mineral, which can be extracted profitably. 3
6.O and a coniferous tree starts expressing views. 5
-7.Something empty with O becomes egg shaped. 5
-8.A region with O becomes a vital layer of the atmosphere. 5
-9.O and a creeper make the adjective form of sheep. 5
-10.O and an exclamation of surprise makes an S-shaped moulding or curve. 4