This is the simplest buttonhole for ribbing. It is shown in K1, P1 rib, but works equally well in 2x1 rib (knit 2, purl 1 on the outside of the garment, which looks like purl 2, knit 1 on the inside). The buttonholes are simply transferred stitches, with the emptied needles returned to work position so that they will knit. This is the same as the simple eyelet buttonhole.
A slightly different method for ribbing eyelets is illustrated below. The resulting eyelet is a little larger..
On the row for the buttonholes, first drop and relatch the rib stitches. Buttonholes are made by transferring a knit stitch to the adjacent needle. Return the emptied needles to position B so that they will knit on the next row.
Knit the rest of the rows for the band, and convert to ribbing. On the stitch that runs down to the buttonhole, insert the latch tool into the hole, go under 2 bars, catch the 3rd bar and pull it under and out toward you, then latch up the remaining bars to form ribbing as usual.
The completed buttonhole.