Is Open Source Software risky for Software Startups?

Open source code has little to do with patent rights - a patent doesn't rely on the underlying code as much as what it's doing, and open-source usually means the code itself, which would otherwise be subject to copyright and some sort of license. So if, for example, you use an open source python routine, it depends what the routine is for (running a machine, networking connection, etc.) because that might be patented, not whether you coded the function in a different language. The main issue with open source is under which license it's been distributed to you - MIT, Gnu, etc. - and your obligations under that license.

This is a question which is independent of whether the origins are open or not. Infringement of software patents has been given a knockback by recent Supreme Court rulings over Alice v CLS. But you might be surprised at how much "patented" stuff actually comes from originally open source, from recollection one content distribution service derived from a proxy cache server released in the 90s. Fortunately if you look at most open-source software, they tend to end up in the tool-chain (for which it benefits other developers), languages (which benefit everyone) and now cloud components. Certain tech giants (eg IBM) have acknowledged the power of the developer community by promising not to assert their patents against bona fide projects (the backlash would be serious ... see Linksys outing) though others have kept silent reserving the right to selectively trip up competitors. As for infringement, that's a matter of doing your homework and either paying for a clearance search or joining one of the defensive patent pools.