A5 clipper sounds as if blade is loose - Oster Golden A5 2-Speed Universal Motor Clipper 78005-140

SOURCE: blade installation

i had the same problem and i fixed mine ( it is not broken) ... the tongue is the thing that is silver and has grooves that face upward, the tongue should be up compared to pressed against the shave, take a small screw driver or a kitchen knife and get under the tongue and lift it up, once this is done you need to slide it on the blades and snap it down, i found that i did not need to have the clippers on to put it back on.... at first i got confused in thinking that the tongue was the red thing at the top, and in actuality this is the thing that moves the blades to clip.... i hope this helps, once you get the tongue up the blades are on in 10 seconds...

SOURCE: Where do I put the oil in my oster a5

across the blades were they contact each other.  WD40 works fine.  

SOURCE: Oster Golden A5 Clippers will not cut

YOU MIGHT NEED TO CLEAN THE HAIR OUT OF THE CLIPPERS. AND OIL IT

SOURCE: can't put on a new blade

start the clippers then slide the blade onto the tongue of the hinge assembly , now push the blade into the clipper don't be afraid sometimes they can be tight from new,
Just make sure the two screws either side of the hinge are tight.

SOURCE: Bought Oster Golden A5 clippers new but did not

There is a small gun-metal colored bar at the very rear, just below the back of the blade. It pushes inward, in the direction of the blade. Push it firmly in and pull up on the teeth of the blade. It should pop upward. You can then remove the blade from the tongue that it sits upon. When you want to put it back, slide the blade back on the tongue, then, WITH THE MOTOR TURNED ON, push the blade down to its seated position. It will click into place.
Terms regarding these devices are all too often assumed as understood. For most purposes, these are what you have to know for the A5 Oster. The cover plate is the part with two screws that has the name and other info on it. If you remove the two screws and the plate, you will see a nylon part that moves the blade back and forth--that is the lever. The other end of the lever, with a hole in it, is connected to a metal part called the link. Both parts are easily removed by simply pulling them upward from the device. The bakelite gear is the part that the link sits on (the larger hole in the link). It too can be simply pulled up off the metal shaft that it rides upon (careful not to lose the washer under it). That gear is turned by the armature gear of the motor (which is where things get much too involved to cover here, but from that point on the device very rarely has problems). Incidentally, the lever, the link, and the gear can all quite simply be replaced. They are available quite inexpensively at Amazon under Oster A5 Clipper parts (note that the one speed and the two speed take different bakelite gears, but the lever and the link are the same). If you were to replace parts such as these or simply lubricate them, remember that a good white lithium grease is recommended in light application for the gear and the shafts (lever and gear shafts and armature gear). Also, the gear shaft has a hole down its middle (and two small holes that lead to the outside of the shaft. The shaft should be filled with grease so that grease can get through those small holes to lubicate the inside of the bakelite bearing. The two pads (one on each side of the lever) should be oiled. The simple way to tell if you need to replace parts is to grip the blade end of the lever while all parts are in place and try to move it back and forth, as though it were running the blade. If there is any more than the slightest movement, replace the lever (replace that anyway if the part that contacts the blade is rounded off) link and gear.
Good luck.