Just as with my Quick Hitter series on the FAR, my Quick Hitter series on GSAR is meant to simplify contracting business with the government. This time with GSA.
GSAR 538.72 Order-Level Materials
An OLM is ordered in conjunction with a TO/DO on a MAS contract to support the specific contract action. It is not an open-market buy. However, an associated OLM SIN is attached to the buy at various stages (from proposal up to expiration) to aid the awardee in supporting the buyer to complete the contracted project in a satisfactory manner.
GSAR 538.7201 General
FAR 8.403(b) gives GSA authority to do business this way. 59 categories in GSA’s MAS program have the capability to have a line-item designated as OLM (T&M). HCAs have designated authority to make suggestions for future improvement.
GSAR 538.7202 Awarding OLMs In FSS Contracts
OLMs are goods or services that are unknown at the time of award, but are purchased to aid in completion of the contract scope. Their necessity may be assumed. However they are not initially included except for the designated OLM line-item which can be attached in the original proposal or through modification after award. These are not ancillary/ODC SINs (e.g. repair). OLM SINs can’t be the only SIN on an award. Price analysis of OLMs is done at the time of purchase and not a part of analysis, negotiation or award of the original contract proposal.
GSAR 538.7203 Administering OLMs In FSS Contracts
The CO can request assistance from the acquisition team to gauge validity of the proposed OLM. OLMs are subject to the TAAs, environmental attributes, the Changes Clause and IFF with exceptions located at GSAR 538.115(d). OLMs must not exceed 33.33% of the award price and the contractor’s pricing strategy and accounting practices are reviewable by the CO and specified members of the acquisition team.
These seem complicated, but if you simplify the given language then they are actually quite simple.
If you think I can help you then email nicholas.s.robertson@outlook.com for your introductory email and free consultation. 
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