There are two denoising options available for rendering with Arnold:
The OptiX™ Denoiser (based on Nvidia AI technology), is integrated into Arnold for use with IPR and look dev. It can be applied to any AOV. The OptiX denoiser is meant to be used during IPR (so that you get a very quickly denoised image as you're moving the camera and making other adjustments). For this reason, it is not suitable for final frame rendering or animations as it can cause flickering between frames (see below).
The Arnold Denoiser can be run from a dedicated UI, exposed in the Denoiser. It favors quality over speed and is, therefore, more suitable for high-quality final frame denoising and animation sequences. To use the Arnold denoiser, you will need to render images out first via the Arnold EXR driver with variance AOVs enabled. It is also available as a stand-alone program (noice.exe).
A denoising tutorial can be found here.
You should only use the OptiX denoiser for test rendering with IPR. The animation below right shows what happens when you use OptiX with animation (flickering occurs). The Arnold Denoiser should be used instead for final frame renders.
Original render using low samples
Creates flickering in animation. Use for IPR only.