I'm guessing this answer because I don't have direct knowledge, but . . .
If it was clean motor oil it will probably do no harm. Transmission Fluid is 10 weight oil. There are usually 12-16 quarts of fluid in a typical transmission, so one cup would be less than 2 percent of the total fluid. Thus, even 30 weight motor oil would thicken the fluid only very slightly.
If you are really worried you can go somewhere and have the fluid changed, and this will be a little expensive. You cannot do this yourself because you can only drain out about 3 quarts. If you do this do not let them do a transmission flush. Only a drain and replace. Flushes can cause serious transmission problems. If the shop insists on a flush, go somewhere else.
Engine oil, even in small amounts can ruin an automatic transmission. You need to have the fluid flushed immediately, and yes flushes are safe.
You need to get the trans fluid changed immediatley
Change oil and filter
Change the oil
You failed to mention what fluid you accidentally put in. In any case, have the transmission fluid flushed out and replaced with the proper ATF right away.
Have it cleaned out and flushed immediately
I am not a mechanic, but I know you have a problem. I would suggest draining the transmission and replacing it with the proper fluid. I wouldn't drive it because I think that might worsen the problem.
Transmission fluid can be poured through the engine compartment on a Honda Accord. There is a dipstick and reservoir where the fluid can safely be placed.
go to a mechanic immediately
Do not pump the brakes. You will need to get all of the contaminated fluid out and refill the master cylinder.
The transmission will fail.
go on bing.com
Before you can change the transmission fluid on a 1997 BMW 540-I you need to remove the plug and drain the old fluid. The new fluid is poured into the top of the transmission in the hole marked for transmission fluid.