TESsnip

TESsnip allows you to open esm/esp files, view the low level record structure and cut/copy/paste records and subrecords between different parents, including different esp's. You can also view and edit the data saved in subrecords, in either hex or string format. TESsnip has no understanding of what data is supposed to be stored in each record, so unless you know the format of an esp file you will be editing blind. TESsnip also supports brute force merging of plugins, which works as long as the two plugins have the same parent masters, don't conflict and only one of the two files adds new items. You can also use it for merging levelled lists, although doing so takes considerable effort, and only works if all items in the levelled list come from a parent master rather than being defined in either esp.

TESsnip cannot edit an ess file. Unlike in morrowind, where the ess structure was identical to esm/esp’s, oblivion’s ess structure is completely different. This means that a different set of utilities will be required to modify it.

The name TESsnip comes from the original morrowind version, which didn't have the ability to move records around, and could only cut whole groups of records out from plugins. TESsnip has come a long way since then, but work has since ceased partly because I've been concentrating on obmm and partly because better alternatives are either under development or have been released. TESsnip is currently most useful as an esp cleaner, to remove unwanted records.

For merging plugins, the best utility that I know of is currently ScripterRon's TES4 plugin utility