Using the February 6+ share numbers, if a second oldies station came into the market and took exactly half of WOGL's 6+ share, they'd have a 3.0. The full market stations with less than a 3.0 (i.e., for whom you can even argue the switch would make sense) are:
WRNB
WRFF
WISX
WPPZ
WPHI
WYSP
Obviously YSP is a sister station to OGL and won't do it.
As for the others, the first four are so close to 3.0 that their demos as compared to OGL's demos (or demos even older than OGL's for "true oldies") have to outweigh the slightly lower 6+ share. WPHI at a 2.3 a) has signal problems, so it's unlikely oldies on there would reach a 3.0, b) also has better demos than oldies would bring, and c) is owned by Radio One, which isn't going to flip from an urban contemporary station to a white-skewing oldies station. It's just not what they do.
Advertisers don't like to try to sell to old people. They don't think it works. If you want old music on the radio, you're going to have to pay for it yourself.
EDIT: I realized I left out WJJZ. Do they count as a full market signal now? I'm not sure if their signal is as good as OGL's, so I'm not sure they could pull a 3.0 as an oldies station.