I would buy at least a couple blades of whatever size you prefer. I personally like a 7F over a 10 because it leaves a little more coat on them, and cuts through the thick hair with a little more ease.
Whenever a blade starts to feel hot to the touch, swap it out for a cool blade. Before you detach the hot blade, spray some lube (see not below about lube) on it and run it for a few seconds.
Whenever the clipper motor gets warm, stop for a bit and let it cool.
Whenever the blade stops gliding through the hair, lubricate it (use a clipper lube like Kool Lube, NOT WD-40). If lubricating and/or cleaning it doesn't help, then consider it dulled and set it to the side. Don't use it again until it gets sharpened.
Read your clipper's manual, especially the parts about lubricating the motor. Some motors need to be lubed, some don't.
Blades also need to be kept clean. While in use, there is blade cleaner that you can dip the blades in while attached to the clippers and running. It helps dislodge all the gunk trapped in them. But you'll also want to occasionally dissassemble the entire blade apparatus and clean it well. DO NOT let it soak in water, ever, because it will start to rust. Be sure to dry it immediately after you wash it, and lube it, too.
There are traveling blade sharpeners in most areas. They usually charge $5-$10 per blade to sharpen them. You can find them by calling local grooming places or vet clinics to ask who sharpens their blades. Generally, the sharpeners won't come to you unless you have a BUNCH of blades, but most will tell you where you can meet them to have your blades done. (Meet them at a local vet clinic or grooming facility) Most places continue to sharpen blades until they will no longer hold an edge or get damaged. It is cheaper than constantly purchasing new blades.....
Oster A5's are an excellent clipper, and should easily be able to handle the task of shaving one dog for many, many years.