How to un-nock an arrow
No you cannot. There is however a mod that you can use which allows you to do this if you are okay with using mods.
You can access the mod here:
http://www.nexusmods.com/oblivion/mods/9333/?
You will need the Oblivion Script Extender, which you can get here:
http://obse.silverlock.org
Confirmed working in Oblivion for PS3:
If you talk to someone, completing dialog un-nocks the arrow. Obviously this may be overly difficult unless you accidentally nock the arrow in a town.