Not just any ork army. Here is some fluff I came up with to tie these orks in with the rest of my collection.
There have been disturbing reports coming in from the fringes of Imperial-Tau space. Aberrant behavior has been seen in human and abhuman (ref.Ogryn-JOVIAN genostrain) have been reported to be manifesting marked xenos (ORKOID) traits.
The xenos tendancies of these remnant settlers is apparent in their ready fusion of IMPERIAL, TAU, and ORK battlefield debris (one can only thank the Emperor these monsters didn't colonize a [REDACTED] tomb world) into functional war materiel. The most disturbing part of the report is the University scribe insists these brutes have adopted the Cult of the Emperor!
Much like ORKOID society, these human tribes are lead by the largest, strongest creatures. The Jovian strain Ogryn seems to be a 'bonehead' (ref.ASTRA-MILITARUM rank designator) and carries an ADEPTUS MECHANICUS spanner-axe. From all the reports returned this monster seems to be responsible for the techno-heresy being performed. The same reports cite a xenos (ref.TAU) drone familiar. The drone has been modified to a nonstandard silhouette and it's function cannot be determined.
As previously stated, the tribes tech is a mix of IMPERIAL, TAU, and ORKOID. A 'DEFFKOPTA' (ref.ORK harrier flier) was seen and picted. It was armed with an IMPERIAL Mars pattern multilaser and crude rockets. All manner of vehicle provide their locomotion in the field.
MECHANICUS representatives from ARMAGEDDON have taken interest in these human tribes due to their xenos traits. They have mustered an explorator force lead by MAGOS [REDACTED] and his ADEPTUS MECHANICUS enforcer troops. They are supported by ASTRA MILITARUM elements seconded from STEEL LEGION and CADIAN xenos specialist units. Also supported by a detachment of TECHMARINE SPACE MARINES in training and diverted from manoeuvers to forge world [REDACTED].
I have more planned for this army, so stay tuned.