I have an oil burning furnace and my hot water only last 2 minutes. Hard to take a shower
From what you say, the general answer is clear: the plumber did something that impaired your hot water. I'm assuming your hot water tank is far larger than 2 minutes of shower would require (let's guess you're using 75% hot water and 2 gallons/minute; that's 2 * 2 * 0.75 = about 3 gallons of hot water in your 2-minute shower).

If we also assume the plumber wasn't messing with the furnace and hot water system, other than to just connect to the hot and cold, I'll guess that he somehow cross-connected the hot and cold lines.

To test this, start the shower and wait until the water temperature falls. Then go down to the hot water tank and check if the water coming out of it is hot (I'm guessing it will be). If the water is still hot, then it isn't a production problem but a transportation problem: somehow the hot water isn't reaching your shower.

You can then follow the hot water pipes and see if you can find a 3-way junction where the temperature suddenly falls. If you find a place where the hot water source pipe meets a cold pipe and a warm pipe, then I'll guess again that the warm pipe is going to your shower, and the cold pipe is pulling in cold water that's diluting the hot water.

No matter what, if the presence of the plumber coincided with the problem, and it hasn't been months or years, call that plumber and tell him to fix it on his dime.