This is just a friendly reminder that even the very best IV analyses can only get us local average treatment effects (LATEs). OLS estimates, on the other hand, are average treatment effects (ATEs). This makes it very difficult to compare the two. See this post by Marc Bellemare for a nice description of the issue. My thoughts: Sure, if you have reason to believe that responses to the treatment will be the same for everyone..or if you believe that everyone in your sample is a "complier," sure, they are comparable. Keep in mind though that either of these instances are likely to be rare. I personally do like to see OLS estimates even in IV papers. But I would urge you to just be careful when making comparisons. The fact that an IV estimate is different from an OLS estimate does NOT necessarily imply that the OLS estimate suffers from endogeneity bias.
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