Car alarm going off constantly

The most common reason for the alarm to go off on it's own is a bad or misadjusted switch. Now as you say the alarm has gotten worse after an oil change the first one I would look at would be the hood switch. In your case the switch is part of the hood latch itself.

Open the hood.
Remove the radiator grille.
Disconnect the hood switch connector from the latch. Now test the system. The switch closes when the hood opens, so if you unplug the switch the system will think the hood is closed and locked correctly. Replace the grille and set the alarm, if it doesn't go off and stays quiet, the switch is faulty. Replacing it means you replace the latch itself.
This method of testing costs nothing but some time. The other option would be to visit a dealer and have them scan the alarm module for the events that are causing it to go off.

To get the correct latch you would need to buy it from a dealer as the aftermarket versions don't include the alarm switch.

Just unplugging it would do that. That is why I suggested it as a test as it would disable the switch from triggering the alarm if it is the cause. There are similar switches in each door lock as well as the rear, any could be the problem but as you say it got worse after the oil change you can start by testing the hood latch. If it stops the alarm going off with it unhooked then it's your choice, the alarm should still work for the rest of the vehicle.