Here is just some ideal, you can turn it into hard-science or fiction as you wish.
Main idea: A creature only Explode when they are triggered (by the creature, or by implant devices). They are not easy to get explode (safety)
- Ex1 Mix-trigger: creature contains chemical A and B. Both A and B are not danger, unless they are mix (triggered). (A= Boranes and B = Boranes , perhaps). But there must be some safety precaution so they are not going a chain reaction (which accidentally kill all in breeding ground)
- Ex2 atom level trigger: chemical A is harmless until exposed to radiation/specific wavelength/a neutron (atomic bomb are triggered by firing a neutron into uranium)
Some problem you concern:
Would a multicellular organism built from mostly boron be able to support life?
You don't need to built organism contains mostly explosive chemical A, B. Let everything be normal as native creature. Then let the creature carry a container of explosive chemical (explosive chemical are not participate in daily life functional, just stock there for last moment). Look at baneling in Starcraft 2. They carry a big bag of green thing.
Would exposure to oxygen be possibly fatal due to the risk of
Solution 1: make A and B not native or battlefield (oxygen for your case)
Solution 2: seal A and B in container that cannot contact with oxygen (in case A or B is oxygen)
How would it respire, if oxygen is a possible danger to it?
Even if A or B is oxygen, the creature can functional if they well-seal their explosive container. You may develop from native creature, then add a biology bag of chemical.
How would it be maintained?
Give them food to eat as you raise sheep. Their DNA can make explosive chemical protein. So you only need to include ingredients in their meal. If explosive chemical only contain C-H-O then everything would be easier.
What would it look like?
From Starcraft 2,
Baneling (ground) and Scourge (air)