Like any other piece of machinery, an OHV is going to require a certain amount of maintenance. Without it you will experience premature wear and equipment failure. The last thing you want is for your dirt bike or ATV to break down when out on some remote trail miles from nowhere. The last thing you want to do is have to push your OHV back to camp! Performing regular maintenance will help minimize the chances of that happening.
Each machine is going to have its own maintenance schedule recommended by the manufacturer. The maintenance schedule will be included in the owner's manual. If you purchased a used machine and didn't get an owner's manual you may be able to find one at your local dealer or on the Internet. In the meantime, there are many routine maintenance tasks you can perform to keep your machine in top shape. You may have to guess about period between major tasks like changing engine or transmission oil or servicing hydraulic brakes and clutches but it wouldn't hurt (except perhaps in your pocketbook!) to do them once a year. Engine oil should be changed more often, especially if the machine is getting lots of use. 2-stroke engines rely mostly on the oil mixed in the gas and don't have oil in the engine pan. 4-stroke engines do have an oil pan and pressurized lubrication systems and you need to keep an eye on the oil level (check it before every ride). If when you check it is gritty or smells burned, it needs to be changed regardless of how recently it has been done. Black oil is OK. Most oils contain detergents to keep the engine clean so fresh oil can quickly become black soon after an oil change. Most OHVs also have transmissions that need attention. Check the oil level before every ride and change the oil according to the manufacturer's instructions or whenever it shows signs of deterioration. Some common problems to look for include metal shavings on the dip stick, which indicate damage to the gears inside, and discolored or foamy oil that may indicate water contamination. Other lubrication should be done as needed. Drive chains should be lubricated before every ride and after they have been cleaned. Wheel bearings and suspension should be lubricated several times a years, more often if your rides include water crossings. Throttle, brake, and clutch cables should be lubricated frequently. Because they are enclosed they probably don't need to done for every ride, but once a month would not be too often.
If you have the owner's manual, or can copy the maintenance schedule from someone who does, be sure to follow the manufacturer's guidelines. Following proper procedures (and documenting it) is often required to maintain warranties and is always a good way to prevent premature wear and tear. Even it you do not need documentation for warranty purposes, it is a good idea to maintain an accurate record of maintenance for your own benefit so you know when it needs to be done again. Good maintenance records may also help improve resale value if/when you decide to sell your vehicle.
Another significant part of routine maintenance is checking for and tightening or replacing loose or missing fasteners. The constant vibration and sometimes jarring impacts OHVs experience causes bolts, nuts, and screws to work loose. Using a thread locker like Loc-Tite will help keep them secure but is no substitute for checking them often (at least before every ride).
Some, but not all, OHVs have electric starters. If yours does, it will have a battery. You will need to monitor the battery condition and make sure is properly charged for each ride. Check the electrolyte level at least once a month and top it off with distilled water if it is low. Use a "trickle charger" to keep the battery fully charged when your equipment is in storage between outings.
Brakes and clutches need regular adjustment to function properly. Brakes that are too tight or clutches that are too loose will slip. When that happens not only do they not function right, they will wear out quickly. Brake pads or shoes can usually be changed by a fairly handy DIY mechanic at home. Clutches often require special tools and techniques, but many riders learn how to do this themselves too.
Tires wear out from normal riding and will often loose a little air while in storage. Check tire pressure before each ride. Some experienced riders use a "pinch test" but using a pressure gauge is more accurate and reliable, especially for novice riders. When tires get worn replace them. Worn tires won't have the traction or durability to support off road riding.
Spending a little time and money doing routine maintenance will pay back many times over in avoiding expensive and inconvenient breakdowns out on the trail. Maintenance may not be as much fun as using your OHV, but its a lot more fun than pushing or towing it when it breaks down due to poor maintenance!
Work it!