This arrangement of "Oliver's Army" is in the key of G, a tone down from the key of Elvis Costello's version, A. Note that the key moves up a tone after the bridge from G to A. In the original key this would give us some difficult chords (B, E, Eb7, F#7, G#m).
In the original recording the start of the chorus "Oliver's Army" coincides with the end of the verse "occupation", which is obviously not possible for a solo performer to sing. Therefore we do a turn to C (IV) then D7 (V7) before the chorus. The same pattern is used in the original recording after the third verse. Alternatively you can cram "yourself an occupation", as in the version by Spunge.
These are the lyrics published on Elvis Costello's web site. Some versions have "Than here today" rather than "But here today" in the chorus.
|Only takes one|
|If you're out of|
|We could send you|