No, you cannot lock the focus point on the D5500. It does not have a focus-lock lever like the D7000 series models do, nor does it have that option in the menu. (It's one of the many little reasons Nikon gives you to buy the more expensive cameras.) Pressing OK after every shot seems like a rather imperfect substitute because sometimes you want to select a point that's off-centre, and you would lose it that way.

However, you can possibly mitigate the problem of accidental movement of the focus point somewhat. The part of the touch-screen display closer to the buttons is called "touch Fn" in Nikon parlance, and sliding your finger over it when the display is off can perform various functions. One of them is moving the focus point, and I believe this is the default setting. I find that I often inadvertently move the focus point around with my nose when that's enabled. If you have the same problem, go to the Custom Settings menu, section f: Controls, and select f3: Assign touch Fn to assign a different function or turn the "touch Fn" feature off. The downside is that you lose that feature this way, which is rather useful. And of course it won't save you from accidentally moving the focus point with the multi selector.