Opening.bnr consists of two parts, an IMET header which is (almost) identical to the latter of part A (starting from Zeroes_1) in [[Content.bin]] (see [[Content.bin file structure]]). However, the last 16 bytes contain some randomly-looking bytes, which is a MD5 sum of 0x40 to 0x640 in the header.
Opening.bnr is also stored in the console specific encrypted area of content.bin (a copy of icon.bin is in the SD encrypted area). For some of the channels a generic opening.bnr (which doesn't resemble the actual banner) is stored in the NAND under the \META tree as TITLE.MET. Their purpose is unknown.