Accidental motor oil in transmission?

You should do a full flush and change the fluid filter in the transmission. While both fluids act as a lubricant the motor oil has additives to make it even slipperier than the transmission fluid and those can absorb into the clutch pack friction surfaces and cause the transmission to slip and cause damage due to the slippage and heat buildup. That transmission is an easy one to service. Jack it up and support it so you can get under it, lay out the gasket and filter you bought so you can compare them to the ones you remove. Now line up your drain pan and start removing the bolts that hold the pan to the transmission. Leave in one front bolt loosely so it catches by a few threads, do the same with two on the rear of the pan. Now gently tap or pry the pan down, warning it will have a lot of fluid ready to spill all over so work gently. Once the fluid starts to flow you can hold the pan and remove the single bolt to tilt the pan down and drain more out. Now remove the pan. Wipe it out and pay attention to the grit or sludge in the pan. That can tell you a lot about the transmissions condition. Now remove the filter. It pulls down while twisting. Now replace the filter seal that is in the hole you just removed the filter from. Install the new filter seal and push the new filter into place. Clean the gasket surface on the pan and install the new gasket onto the pan. Now normally you would torque the pan bolts in a cross pattern to 97-inch Pounds. However, as yours is contaminated just tighten them snug for now. Now go to the transmission cooler lines and remove them at the transmission end. Put the end on one into the drain pan and use air to push the fluid out of the cooler circuit. Reinstall the lines.
Now fill the transmission with enough Dexron 3 fluid that it shows on the stick. Start the engine, shift through all the gears and let the trans run in each for a couple seconds. Shut the engine off. Remove the pan again and drain out that fluid. Now you can put the pan back on and torque the bolts down and fill the transmission fully. It should have removed 90 percent of the bad fluid from the transmission and converter. Just pay attention to any changes like slippage or strange smells. The second part of this covers the pan and filter style similar to yours.
https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

The cash issue seems to be a universal problem this year. That and the lack of parts. That is one of the issues with motor oil vs transmission fluid in terms of usage. At least it isn't motor oil in the brakes or clutch. Those can cost you serious money because the additives and base oil stock will cause all the rubber parts to swell and soften and the only cure is to replace all the rubber lines, the master cylinder, ABS module and calipers or wheel cylinders. Plus flushing the lines with a solvent to be sure any petroleum products are out of the system.
In your case as long as you get it out of there and don't run it, other than to flush the converter it should be okay as long as the transmission isn't already worn badly. Normally at 99K it would have had the fluid and filter changed out at least once and this would be the next normal change, if you drop the pan and find it's full of grit or black flakes you can still change the fluid and filter but it's on borrowed time at that point because those things are the friction surfaces of the clutches and bands in the transmission and it becomes a toss-up as to the remaining life of those. That is why some folks will say "I had the fluid flush done and now my car won't move" The particles suspended in the oil were actually acting to hold the friction on the parts. The flush cleaned that all out and now it slips.

As long as you get it out of there you have a better chance of not causing problems. That transmission is a pretty tough unit and might survive with clean fluid. Hard to say with transmissions. My general rule of thumb is that if the vehicle has close to or over 100K and the transmission wasn't serviced to leave it alone and just run it to failure and rebuild it then. The issue with the engine oil are the anti-friction additives, they are great for engines where you don't want friction, but not good in a transmission that has friction parts in it.