it all depends on what kind of oc you want to achieve.
when talking about the 2600, people usually meant oc'ing it to a 2600x level that they can run safely 24/7, i.e. 3.9-4.0 ghz, which the b450 will support.
when talking about the 2600x/2700x, people meant the max it will do under load, i.e. 4.2ghz all cores, 4.4ghz single core, which requires the x470 chipset and a better board.
the others are right too. i'm not trying to influence your decision, just dumping info on pros and cons.
i'm not saying if you want to oc pick the 2600x. i'm picking it based on the options you gave, hoping that it'll do 4.2ghz at lower voltages. because there's a chance that a 2600 would max out at 4.0ghz.
but if it's up to me, i'll pick the b450 with 2600. because i prefer lower temps and power consumption, and would be happy with 4ghz.