(ฅ^•ﻌ•^)ฅ CAT DADDY ฅ(^•ﻌ•^ฅ)
It's not exactly a surprise party -- he makes his intent pretty clear when he calls Kuroo to come over (at afternoon, to make sure he doesn't complain too much, although it's not the other's usual waking hour yet, anyway); the reason for the visit itself is left out, however, as he purposefully fails to specify one. Secondly, it's hardly a party at all -- just Oikawa, wanting to be nice, for a change (that, and he's selfish about the time he gets to spend with Kuroo; it's limited as it is, so he's not exactly willing to share.)
Picking out a serious gift proves to be troublesome, as Kuroo isn't exactly a very material person; instead, Oikawa gets him a small cake, stocks up on his favorite booze and buys some joke presents (they're wrapped up nicely, as if they're luxurious, but, really, it's just meowmix and something he found in a five dollar store.) If Kuroo wants anything in particular outside of this, he can always speak up -- it's better to wait rather than deal with a failed present, anyway (he's fully prepared to ask, too, but first, he wants to see Kuroo's face when he opens the current gift.)
He leaves the door open for the other, and as soon as Kuroo arrives, Oikawa's there, stuffing the wrapping into his hands with a big grin on his face. "Happy birthday, Kuro-chan! Or, rather, happy I-can't-believe-you-can-be-considered-an-adult-now day."