I dropped the oil and changed the oil filter the other day when to ride my 250ktm sxf and there was a large loss in power any ideas?

When was your last oil change? Was the oil filter correctly reinstalled. I'd take a run into a service shop and have them take a quick look to see if it is something simple.

if its only since you changed fuel filter it may be not sealing properly and getting air in ,,,, sounds like a missfire though

You seem to have missed the air filter.

Hey Anon, Run a compression check..screw the compression gauge into the head until the o-ring seats..( a coat of oil on the o-ring helps) hold the throttle wide open and kick it over about 6-8 times HARD.. you should get at least 120psi..if it is lower than 110psi you need to find out why...
if you don't have the tools/knowledge/experience .. .. take it to a qualified pro...

check clutch idler pulley, it may be froze up and causing the motor to drag down

Is the air filter blocked with oil dirt or debris?

Take the prefilter off and see what shape its in...if you have one!

Otherwise, you might just try a new filter. If you drop the filter on the ground squarely on its lower edge sometimes you will see very fine dust come out in large quantities. This would be a good sign you need a new one.

You are in need of a new oil pump, drop the oil pam off and replace the pump, DO NOT DRIVE THIS UNTIL THE LOW OIL PRESSURE IS REPAIRED OR YOU WILL NEED TO REPLACE THE ENGINE

Could be low on compression, or torque converter may be slipping

Fue/filter blochage