In the latest Commons register, Mr Field declares he is a paid director of Medicash Health Benefits Ltd, a private health insurer, and his regular
column for the magazine Pensions Week, earning him "up to £5,000".
The former welfare minister is also a prolific writer of articles for a variety of national newspapers and magazines and makes occasional, paid appearances on BBC Radio.
Mr O'Brien, meanwhile, tops up his salary with two directorships – of Eden Research plc, an agrochemical development company, and of engineering company 3DM Research Limited.
He also earns £15,000-a-year as an adviser to the Institute of Chartered Secretaries and Administrators, and a further £15,000 advising the Manufacturing Technologies Association.
The requirement on MPs to list hours spent on second jobs – part of reforms prompted by the expenses scandal – was originally opposed by the Tories, which feared the revelation of hourly rates would be embarrassing.
However, the issue of outside jobs has also been controversial for Labour.
Since 2006, thirty-seven former ministers have accepted private sector jobs within two years of leaving office. The new rules come into force on Wednesday, but will not immediately trigger a flood of new information on the register, which is published on Parliament's website.
They are not retrospective, only requiring an MP earning after July 1 to register that income, the nature of the work and the hours worked within 28 days.
