No you don't need a job offer to apply for a open work permit.
The issuance of the province-specific open work permit will not be conditional upon the nominated applicant having submitted an application for permanent residence, based on his or her nomination certificate. The issuance of the nomination certificate itself will allow the foreign national to apply for the work permit.
Eligibility – Applicants must meet the following criteria to be eligible:
-The applicant must be a participant in the FSW Backlog Reduction Pilot.
-The WP application must be accompanied by a letter of support from the nominating province that:
confirms the applicant is being nominated under the FSW Backlog Reduction Pilot; and
indicates the WP will only be valid for work in the nominating province.
WP issuance and extensions
A province-specific, non job-specific open or open/restricted (depending on medical examination restrictions, if applicable) WP may be issued to the eligible applicants as described above upon receipt of a WP application and letter of support from the nominating PT (as described above) without requiring a Labour Market Opinion (LMO) from Service Canada.
An applicant may apply for the WP at a mission, Port of Entry or in-land.
The applicant must also meet all other admissibility requirements to be issued a WP under the FSW Backlog Reduction Pilot.
WPs issued under the FSW Backlog Reduction Pilot may be valid for a maximum of two years.
Because WPs issued under the FSW Backlog Reduction Pilot are intended to facilitate faster labour market entry while Permanent Resident processing is underway, extensions beyond the initial two year validity period will not be granted.
Source: http://www.cic.gc.ca/english/resources/manuals/bulletins/2012/ob470.asp
Much like post graduate work permit(open), you don't need a job offer to get one.
