The third day of the federal racketeering trial of former Probation Department boss John O’Brien opened with US District Court Judge William Young turning aside a bid by O’Brien’s lawyers to grill a former Probation official about the details of hiring after O’Brien’s tenure. It took pit stops in an effort by Sen. Marc Pacheco to help a Taunton probation officer jump to the front of the promotion line, an alleged bid by Senate President Therese Murray to bump up a state worker’s pension by transferring her into a more lucrative Probation job, and veered into the patronage practices of the Roxbury District Court during the 1970’s.

And by the time it was over, Young was scolding the prosecution from the bench, telling the top public corruption lawyer in the US Attorney’s office that the skirmishes he and O’Brien’s lawyers are engaging in have had virtually nothing to do with the federal crimes that the O’Brien trial is supposed to be weighing.

“This is a case about mail fraud,” Young told Assistant US Attorney Fred Wyshak Monday afternoon, after jurors had filed out of a Moakley Courthouse courtroom for the day. “If racketeering grows out of the relationships of the mail fraud, so be it. If the conspiracy is a conspiracy to set up, to cover up a patronage hiring scheme, so be it.”

Young appeared frustrated that lawyers for both sides have spent the first three days of the O’Brien trial largely trying to litigate the qualifications of politically connected Probation hires and promotions that O’Brien allegedly engineered. “The government has to prove … false representations of material fact,” Young said at the end of Monday’s session. “This is not a case about patronage hiring. It’s simply not.”

Prosecutors have charged O’Brien and two former deputies, Elizabeth Tavares and William Burke, with racketeering, conspiracy, and mail fraud. They’ve claimed that O’Brien ran a rigged hiring system in Probation, one that claimed to be making legitimate personnel decisions, but was actually trading jobs to lawmakers for increased funding and political chits. During opening arguments last week, Wyshak claimed O’Brien “handed out jobs like lollipops” to well-connected applicants.

During the trial’s first three days, lawyers for both sides have wandered far afield, sparring, for instance, over how many members of the well-known and politically wired Bolling family worked in Roxbury District Court four decades ago. Patronage is enmeshed with O’Brien’s alleged mail fraud, but they’re not one and the same, and both prosecution and defense lawyers have appeared to have trouble seeing the forest for the trees. So on Monday, Young repeatedly yanked the leash, trying to get both sides to the actual crimes at hand.

Young indicated Monday morning, before the jury entered the courtroom, that Wyshak had been correct when he claimed, during his opening arguments, that the federal crimes rested in what O’Brien and company had to do to cover up its allegedly rigged hiring system, not in the patronage hiring itself. The trial is “about cover-up, nothing more, nothing less,” Young instructed.

Later, when lawyers wandered into the weeds again, Young stopped testimony and addressed the jury: “You’re not being asked to be, and you were not set up as, a super-personnel board. That’s not what this case is about,” Young said, repeating what he’s said numerous times already. “You’re not engaged in the business of Probation, and neither am I.”