What 29 CFR 1904.30 requires (plain English)
A contractor with a main office, a yard and six jobsites has one question the day someone gets hurt: which log does this go on? 29 CFR 1904.30 answers it. The basic rule is a separate OSHA 300 Log for each establishment expected to be in operation for one year or longer (1904.30(a)). The definitions section fills in what an establishment is: a single physical location where business is conducted or where services or industrial operations are performed. For work like construction, where employees don't work at a single physical location, the establishment is the main or branch office that supervises the work or that crews work from (1904.46).
OSHA gave its own construction example when it wrote the rule. A company working in several states might keep a log per state for its short-term projects, plus a separate log for each construction project expected to last more than one year. A company with one office location and no project lasting more than a year needs only one log (preamble to the recordkeeping rule, 66 FR 6035-6037, January 19, 2001). Short jobs never drop off the books. 1904.30(b)(1) requires records for them and only lets them share a log.
Two more paragraphs keep cases from falling between logs. Every employee is linked to one establishment for recordkeeping, including people who move between sites or don't work at any of them ((b)(3)). And a case goes on the log of the establishment where it happened when it happened at one of yours, or on the log of the establishment where the employee normally works when it happened anywhere else ((b)(4)). The preamble spells out the case that matters on a shared jobsite: an injury at another employer's workplace, or in transit, goes on the log of the employee's home establishment.