I am not familiar with Oster A5 clippers but having looked them up I see that there are two screws on either side of the body. These on other makes of clippers contain what are known as the brushes which are an intregal part of the engine but they do wear down after a while.
If you unscrew them, there will be a spring holding the brush (which is a hard stone like material) and you will see if they are worn out. Easy to replace and cheap but, you might have worn into the engine in which case it is more expensive repairing them.
I use WD40 on the blades when I am clipping. Turn the clipper sideways and spray so the oil goes between the blades and, after spraying, use a cloth to wipe off the excess.
One of my favourite clipper is the LAUBE, basically a dog clipper either mains or battery. They are capable of doing a full clip on a big horse and the width of blade is inter changeable. (Ostler blades fit them too)
I like them because they are light, quiet and as I do a lot of difficult horses there is no wire to worry about.
They are also made in the US.