I have made many small improvements to my OSSAs over the years. Here is a partial list of things that have not already been mentioned. These are listed, more or less, in decreasing order of importance. Photos are in the image carousel.
Fast throttle tube. This should be self-explanatory.
Wider footpegs. Oh, the comfort and control!
Handlebar raisers. I made mine about 15.5mm tall. I know a lot of top riders discourage their use, but they are great for this old man.
Rear brake lever extension. OE is tucked too far inboard for my liking.
Bar-end sliders. Although these are common on trials bikes, I made my own from acetal plastic. The design comes from my supermoto bikes and uses a retention mechanism similar to that found on Bark Busters. They are great to avoid a sticky throttle if you plant the bar in the dirt.
Progrip 699 Superbike handgrips. I use these same grips on all my bikes. They are very thick and fairly comfortable. Also visible is a white plastic disc that keeps the handgrip from rubbing against the throttle tube housing.
Hourmeter/tachometer. Mine is made by Trail Tech and includes a tachometer with maximum-RPM telltale. I use it mostly to gauge fuel usage. The photo shows the old silver model. I love the upgraded model that offers more functionality including a time-of-day clock and resettable hour meter in addition to the non-resettable hour meter. The max RPM telltale now holds that value until reset.
Clutch cover dowels. I replaced the split spring-locating-dowels with “solid” ones from Yamaha (P/N 99530-10114-00). See photo in Image Carousel. This makes removing the clutch cover easier.
Floating front rotor. The front brake on my 280 squeaks and I thought making the rotor float would help, but it really does not. The front brake on the 280 is more powerful than either 250, despite all of them both having my favorite (EBC) pads.
Fuel level sight-gauge. This was something I should not have needed, but before I got the 280's EFI sorted it was a huge help in not running out of fuel. There are photos in the Image Carousel. I now rely on the hour-meter instead.
Check-valve vented fuel cap. The tiny OE fuel cap is a PITA. It is made tolerable by removing the dangling vent hose and replacing it with a motocross vent cap. Make sure you get one that actually has a check valve inside (it should rattle). I mistakenly bought a cheap batch of counterfeits via eBay that did not.
M12 x 1.5 oil drain extension. Both the fill and drain plugs use an M12 x 1.5 thread.
Jitsie filtered airbox cover. Only on the 2011 bike because of the difficultly cleaning its OE filter. It does affect the fueling.