Mac OS version, OS X El Capitan, macOS Sierra, macOS High Sierra, macOS Mojave. Either way, you need to decide if you want to save a bit of money on 10.5 or get iWork and iLife as part of the package. The compatibility table for End-of-Life Parallels Desktop versions is below. My current Mac came with 10.5 so the upgrade to 10.6 was cheap.
I was a ludite happy with OS 9 on a Wee Pismo. I saved $$$ when I bought retail 10.3 and 10.4 disks. I see someone is selling gray disks from a iMac! Buy that. The ones advertising as 'Retail' tend to run on the higher end. Just scanning eBay you will find 10.5 running from ~40 to ~200. Mac OS X version 10.4 Tiger, is the fifth major update to Mac OS X for Apples Macintosh. You are essentially paying ~140 for 10.5 plus the added goodies of iLife and iWork. If you want to save $ you may be able to get 10.5 from a dealer-RETAIL DISK-say on ebay, but if you poke around HERE you will find that often results in people buying machine-specific DVDs which become glorified coasters. I want to upgrade to Mavericks, but I cant do so without using the Mac App Store to download the upgrade. Apparently, you can but it is more expensive. Im running Mac OS X 10.4.11 on a 2007 MacBook. Salsoulkid I did not think you could unless you already had 10.5. According to my Mactracker, the highest OS is 10.5.8.
