An OTS spokeswoman confirmed that Dunmore was working there, but said that he was not a consultant. She declined to say what he was working on and said she could not comment on whether he was being paid.
"Stephen Dunmore is supporting the OTS in an advisory capacity because of his significant experience in the sector," she said.
Dunmore spent more than a decade at the BLF and its previous incarnation, the New Opportunities Fund.
Vanessa Potter, former director of policy and external relations at the fund, worked as a consultant at the OTS immediately after leaving the BLF in January last year.
Potter's work at OTS involved a three-month strategic review of the department.
Helen Stephenson, formerly head of strategic policy at the BLF, was appointed deputy director of third sector support at the office in May last year. She was one of four deputy directors brought in as part of a departmental restructure.
Both the strategic review and the restructure came after the establishment of the OTS by former Prime Minister Tony Blair in 2006, when former NCVO public policy director Campbell Robb was appointed its first director general.