So I didn't read the description on eBay and bought clip on aero bars. The aero bars are meant for MAX OD 26mm. I have a 31.8mm bar. Is there any way to adapt to the 31.8mm?

You can try drilling the clamps to ~32.5 mm, maybe a little less. Clamping bolts will have to clear the drill bit, of course.

Brad

Simple answer: No. You bought the wrong aero bars so you should buy the right ones. I can't see it would be safe to drill, Dremel or file out the clamps to fit your current bars.

I'm not sure what Brad is recommending with the 28.5 mm enlargement as that diameter fits nothing.

Major typo!!! Fixed, but no idea why I typed what I didn't mean to.

Brad

Your 31.8 bars will likely have a raised portion in the center, and then drop down to a thinner section (less than 26.0). Do the bars have aero wings?

Anyway, perhaps you could figure out how to shim your bars up to 26.0, and clamp beyond the raised section.

Best bet, however, is probably reselling your clip-on bars and buying the right ones.

Anything is possible with a torch....

Like CliffordK said, your bars are 31.8 mm at the stem clamp. Most bars have a center bulge where they're clamped by the stem and they'll taper down outside of that bulge, typically to 23.8 mm since that's the diameter that road bike brake levers and integrated shifters are designed to work with.