Multichannel EXRs are possible using Merge AOVs.

Compression

Specify the type of compression used.

Half Precision

Specify whether 16-bit floating-point (binary16) is used (as opposed to full 32-bit precision). This provides a sufficient degree of precision in many cases while keeping data usage down.

Tiled

Preserve Layer Name

If checked, the data of the different AOVs will be stored in the EXR in layers with the names of the AOV, and not the RGB.

Autocrop

Append

Merge AOVs 

This option saves multiple AOV layers into one multi-channel EXR file. It writes all enabled AOV's to one image file, naming the layers in the EXR with the name of the 'RenderPass'.

Metadata

It is possible to write out custom EXR metadata using integer, float, point 2, matrix 16, and string values.

EXR overscan options can be found in the Render Settings.

Multipart