AFAIK, it's a senpai-kouhai one. Both were training ships before Ooi was converted into a torpedo cruiser.
But why recently? The Mitsukoshi collaboration might has something to do with this.
Let's go to an amusement park this Sunday, shall we? Ooi-san.Eh?Eh?Of course you'll——come with me, right?Weeeell, I have a plan to go with Ooicchi next Sunday, y'know? Just the two of us.