“Oil field trash” is a state of mind for the various types of people who work in the upstream part of petroleum production. While usually highly skilled, well traveled and wealthy, many oil field workers associate themselves strongly with blue collar work. They work long periods at a time with erratic schedules, and a maintain a sense of normalcy which appears alien to those looking in from the outside. They greatly look down upon traditional desk work and assume, expect, and hope to receive a reciprocal feeling from the nine-to-fivers.
Additionally, their recreational time is also differently spent. They don’t have regular weekends to look forward to, so they enjoy themselves when they can. Often rowdily. A common oil field expression is "Work hard, play hard". After spending weeks on an off-shore platform, spending a few days blitzed in the local pub seems a nirvana of sorts. This combination of a need to release, money, time off, and rowdiness leads to great excess which to the normal folks might be a bit frightening.
Hence they preemptively call themselves "Oil field trash". A warning however, it's only used within the guild. I strongly don't recommend walking up to your local roughneck with an opening line of "I thought they didn't let oil field trash in here?"
The result could be messy.