Obstacle-free sector is a complex surface originating at, and extending from, a reference point on the edge of the FATO of a helideck, comprised of two components, one above and one below the helideck for the purpose of flight safety within which only specified obstacles are permitted.
Source: IMO Resolution A.1023(26), Code for the Construction and Equipment of Mobile Offshore Drilling Units (2009 MODU Code), 2009. Global Standards
Obstacle free sector is the take-off and approach sector which totally encompasses the safe landing area and extends over a sector of at least 210º, within which only specified obstacles are permitted.
Source: IMO MSC.1/Circ.1431, Guidelines for the approval of helicopter facility foam fire-fighting appliances, 31 May 2012, International Maritime Organization, Regulatory guidance