Brennan's admissions to Racing Victoria stewards included that he administered "vitamin complex" in an intravenous drip to horses trained by O'Brien and Mark Kavanagh last spring.
He said the two Melbourne trainers paid him $3000 for the substance, which Brennan alleged he forwarded as payment to another vet.
Mark Kavanagh has denied paying Brennan, while O'Brien has disputed some aspects of Brennan's evidence concerning the payment. Brennan's evidence to Racing NSW and Racing Victoria stewards was that he did not knowingly administer cobalt.
He says the vet who gave him the bottles of "vitamin complex" assured him it contained no prohibited substances, including cobalt.
Racing NSW have since interviewed the vet named by Brennan and are continuing to pursue that line of inquiry. Victorian stewards are also looking into the matter further.
Brennan has agreed to stand down from practising on racehorses from August 1, pending the determination of charges against him.
He has been charged by Racing NSW stewards with 11 cobalt offences with the matter to be heard as part of the Midsummer Sun inquiry which will be held next month.
The Sam Kavanagh-trained Midsummer Sun tested positive to cobalt and caffeine after its Gosford Cup win in January.
Sam Kavanagh will have 24 cobalt charges against him heard at the same inquiry next month.
Brennan, O'Brien and Mark Kavanagh have been charged by Racing Victoria stewards in relation to cobalt positives returned by horses trained by O'Brien and Mark Kavanagh but no dates for their hearings have been set.
- AAP