Open Collective (really Open Finance Consortium Inc.) is a US 501(c)(6) nonprofit that runs a payments and accounting platform, providing fund acceptance and budget services to a ton of different community collectives and funding groups, making it easier to connect funders with groups that are often not incorporated.

Open Source Collective is a separate 501(c)(6) organization that actively supports funders wanting to support FOSS projects or communities specifically. They share some board members, and they simply use Open Collective to do all the finance work, while also offering some level of advice and other IP holding services: https://docs.oscollective.org/welcome-and-introduction-to-os...

Open Source Endowment is different, in that it's soliciting 501(c)(3) donations, which the OSE board and membership will use for the endowment to choose FOSS projects/communities to provide grants for.

This topic should be a FAQ page on the OSE site, especially for funders who just want to donate "to some good FOSS" without knowing where to find it. When you donate to OSC, you pick specific collectives to give to (and it's not tax deductible). When you donate to OSE, you're giving to the endowment, that the OSE Members setup policies for how/where/when to provide grants to projects/communities (and it could be tax deductible).