can someone explain why chi o is ranked below dg?

You can't base your entire understanding of sororities from their Instagrams lmao. This will not give you an accurate read of campus reputation or member experience, especially at a school like Cal Poly where the student population is generally very attractive. You also have no idea how different houses' marketing departments work. Some will literally only post their most attractive members to ensure a certain public image is portrayed while others will try to represent everyone. You won't have an accurate understanding of the "ranks" until you get on campus (besides MAYBE a general idea from GreekRank discussions).

To answer your question, ChiO has a reputation on campus for basically taking everyone who gets dropped everywhere else (though slightly more selective than theta). They have a difficult time with fraternity relations which deters a lot of members because exchanges and co-ed social events are a HUGE part of greek life whether people admit it or not. They only land a couple of exchanges per quarter with low tier fraternities which I think is a big part of their reputation issue. The sisterhood seems fine, though they don't really have a solid vibe because they take such a mixed bag of girls.

On the other hand, DG has a way more cohesive vibe and significantly better fraternity relations. They are known as being studious but still down for a good time and typically have a similar social calendar to houses like KKG and GPhi (way more fraternity exchanges and just events in general). This is a controversial take but I have always thought of them as above (or at least equal to) KKG, but I am also entering my senior year so that may be why.

This is all coming from a girl in a sorority traditionally considered top 3 so I am a little detached from these houses but I will say I am friends with a lot of people in DG and virtually no one in ChiO. All you really need to know is that every house except for ChiO and Theta generally offer the same night life experience, it's just a matter of who you get along with more. DG definitely differs in social life from AXO/APhi/SigKap but it is still a very solid option and you will have events to go to. I can't say the same about ChiO.

Bottom line, just go through recruitment and see who you get along with the best because between the vast majority of houses, your experience will not be that different (this isn't SDSU you can go to whatever parties you want). I would be wary of joining Chi O or Theta though unless you don't really care about frats just because it is a different social situation in terms of your private exchanges. Good luck I hope this helped!