I did an oil change on my subaru legacy 2000 and now it's starts but once it's in reverse or drive it wont [email protected]

How many miles do you have on the car? Are there any leaks? Are there any funny sounds when you try to start the engine? Any knocking sounds?

Try removing the shifter cable from the shifter arm on the transmission . Remove the arm from the shaft on the transmission and position the arm 180 degrees and install it back on the shaft. reconnect the shifter cable to the arm and see what is does then. Outside of that I don't know.

e: subaru legacy transmission dipstick Transmission dip stick is behind the engine in front of driver side firewall. Move a few hoses out of the way, look down and you'll see it has a yellow top.
Other side (passenger) lower down yet, again move a few hoses aside, yellow top handle, you'll find the shorter differential dip stick.
In either case, if your gaze reaches the ground, you've gone too far.

I would check the transmission fluid level to make sure it's full. If it's full and still doesn't move you may have to take it to a repair shop.

check the level of fluid... most gearboxes take a second to gain oil pressure so if there is low oil because of a leak etc... it may cause a few gears to not select imediately after starting

Well in a auto gear box you have in the pan on g/box a filter and a forward oil pump and a reverse pump also a set of bands forward and a reverse plus a valve body assy which has a load of small pistons and springs and oil ways ,but by the sound of your problem is that the forward pump isent going through the body valve for some reason so its taking a short cut and bypassing 1,2,3, and trying to to top first.
So sorry to tell you that the answer to your problem is you have to go to a Auto Matic gearbox shop and they will have to fix it ORget a second hand box and fit it yourself.
Let me know how you go. are you on Skype Ron

First Did you take the emergency barke off? and have you checked the fluid level? even a pint of fluid can damage transmissions. look at fluid is it brown? if it is you need to change the oil and filter in the transmisson. Does it go in reverse?

need to get trans looked at no simple fix without looking to diagnose problem,dont drive it till its seen to as it could be serious

your transmission could be getting weak or the module on the side of the transmission needs replaced

The Dipstick is on the driver side right next to the starter and yes you also put the fluid in there.