It's not code compliant and you risk a house fire. A microwave pulls a lot of amps and is not designed to work with an extension cord. Call in the electrician and have him run you a completely new circuit from the box to the cabinet above the range. As mentioned, they pull a lot of amps and putting them on a small appliance circuit means you risk flipping the breaker occasionally when you use the MW and other high wattage appliances at the same time.

Right now I have the OTR microwave plugged into a wall socket and have been using it for a few days and none of the breakers have tripped. So why would it be a problem if I plug it into a wall socket in the kitchen.

The extension cord would have to be rated for at least 15A or whatever the MW's maximum amperage is, like those designed for air conditioners, but it's still against code as it could start to come apart if, say, a metal pan handle is pushed into an upper cabinet and it lands between the two connecting parts. Another issue is that a built-in appliance normally requires a dedicated circuit - one not shared with other outlets and electrical devices. The outlet you're running it to may be on the same circuit that the toaster plugs into, overloading the circuit if both are used at the same time.

I have to install a dedicated circuit for a microwave oven. I was wondering as to when I should pull a permit for this work? Should I do that before I have the work done or after? Also what are the consequences of doing it without a permit other than it being difficult to sell.
Had another question with respect to installing a dedicated circuit vs pulling from an existing 20 amp line. Right now I have 4 outlets on that line. If I plug in 4 appliances that draw more than 20 amps wouldn't it trip the breaker. How is it different from operating the microwave?

Just call an electrician. If a permit is needed in your jurisdiction, he can pull it and call the inspector to come and take a look.

My microwave is on the same appliance circuit as my coffee maker. I can't make coffee and heat up anything else at the same time.

The permit should be before. Not getting a permit depends on your jurisdiction.

Question for weedmeister. How did you get a permit if your microwave is in the same circuit as the coffee maker. My neighbor did the same thing in fact he just ran an extension cord from the outlet to the microwave and his breaker never trips.

Weed is probably talking about a counter top MW which is plugged into a counter electrical circuit. You don't need a permit for that. However, as a KD, for the last 20 years, I've personally spec'd a separate dedicated electrical outlet for any MW in a kitchen, regardless of it being a counter MW or the already required one for a built in like an OTR. That way, you can use the coffee maker and MW or toaster oven and MW at the same time and won't need to go into the utility room and flip the breaker and then pick one over the other to use.

Your neighbor is an idiot. Do not use his getting away with idiocy as something to aspire to. There are plenty of creative and misguided wrong ways to do something and get in trouble here. But there is only one right way. Just call the electrician and be done with your problem.

I have felt and seen extension cord sockets get so hot that they smoke. Follow the NEC codes and have the job approved by proper officials. It is to protect you and your family. Otherwise, if someone gets killed, I believe that a firechief has the authority to arrest and the insurance company will have a reason to deny.

A microwave oven that actually pulls 15A should be on a dedicated 20A circuit.

In the world of electrical permits, the maxim: "better to ask forgiveness than permission" is not valid. Permit first. Whether you can pull one will depend on your local code enforcement, as determined by state law. In NH, you have the right to do your own electrical and obtain a permit therefor so long as you own and live in the residence, it is single family, and you comply with the National Electrical Code (NFPA 70). Permits are required if you pull any cable, but not if you replace a switch. There is a lot to know; even being an electrical engineer is not sufficient without studying the Code.

I strongly recommend that you (a) have a working knowledge of electricity, (b) have a copy of the latest NEC Handbook (available from the NFPA probably about $180), and (c) read all of the sections relevant to your planned installation. For many not versed in (a), or lacking enough time for (c), it will be more cost effective to hire an electrician.

The NEC Handbook does not teach electrical safety, so this must be understood first.

Various tools are needed.