I have just begun to experience strange behaviors with my bow occasionally performing a sort of rapid fire while I attempted to zoom (holding left and right mouse buttons simultaneously).
At first, I assumed it was because my mouse battery was very low, but a fresh battery does not improve the performance.
Just now, also, I found that I can ready my blade or blunt weapon, but I cannot swing it to attack.

I have full fatigue; I can see no harmful effects against me, nor any damaged skills or stats.

I am at approximately 400 hours logged in my current game.
I only recently overcame the slow animation bug via recommended Oblivion Animation Fixer. (Thank goodness for small miracles!)
I have never used any other mods. I am running Oblivion on PC with no additional content. I recently downloaded the recommended English patch for PC.

Thank you for any advice or insight.
-sols

Do you have a joystick or other device installed on the system ? If so try disconnecting it (might be getting random signals from it even if it is not in use) -- if that is not it then perhaps try a different mouse and see if perhaps your mouse buttons are getting worn out ( or change the key setup so that it uses some other key combination )

Thank you for the suggestion.

I have only the peripheral usb wireless mouse attached. It has always been effective, until now.

Previously, if my weapon was sheathed, one left-click would draw (ready) my weapon, and a second left-click would strike.
Currently, the first left-click will draw my weapon, but successive clicks have no effect. The weapon arm just twitches a bit, randomly, as it holds out the weapon.
Right-clicking continues to raise my shield to block.
This suggests to me that my mouse is functioning properly, though I am not an expert.

Additionally, I had previously mapped keyboard keys for both block and attack. The keyboard-attack was no more successful.
(As a side-note, I have never been able to use Remote Manipulation spells to effect anything outside arm's reach. I have simply flailed my arms about. Explanation of proper technique would be helpful there, too.)

After searching the forums, I did try to confirm/correct my oblivion.ini file. I did not know specifically which values applied to my mouse, though I did confirm the default joystick values, as posted in other threads.

My laptop does have a built-in mousepad, with left and right buttons, and occasionally it adds unintentional pointer movement if I touch it. But that has not caused problems while playing Oblivion. (I have not tested the built-in mouse right and left buttons.)