Abstract: We study the effect of within-occupation employer concentration and outside-occupation job options on wages in the US, identifying outside-occupation options using new occupational mobility data from 16 million resumes. Using shift-share instruments to identify plausibly exogenous local variation, we find that moving from the median to 95th percentile of employer concentration reduces wages by 3.9% on average and by 11.3% for the occupations in the lowest quartile of outward mobility -- those for whom the local occupation is a good approximation to their true labor market. We also find meaningful effects of changes in the value of outside-occupation job options on wages: an exogenous 1 percentage point higher wage in outside option occupations leads to a 0.1% higher wage in workers' own occupation. Our findings imply that employer concentration affects a non-trivial minority of workers, that policymakers should take the effects of employer concentration seriously for these workers, and that when identifying labor markets where employer concentration may be a concern, measures of employer concentration within an occupation should be considered alongside occupational mobility and the availability of outside-occupation options.