Are people allowed to wear OCP bottoms with a civilian top?

I'm retired, so.... I basically wear what I want unless on post. When it comes to the ASU, I don't wear that right now because I haven't had occasion to attend anything worth shaving my beard. If I wear the ASU I will shave. Other than that, I don't worry about it.

Sir, it depends on what “civilian tops” are. As mentioned in the 670-1, “not exclusively military in character,” can be a gray area for unit t-shirts, hoodies, etc. It depends on the garrison/post command policies, but within the two stations I have been in, as long as they were unit-approved apparels, we were able to wear “civilian-like” clothings for PT, ruck, and during organizational days. Every military post and unit is different so this is a difficult question to answer.

“Ruck scheduled this week”. If the commander directs that to be the uniform, then “that’s” the uniform. End of story.

You’ll figure out most of AR-670-1 has the phrase “or at discretion of commander” for most portions. Meaning the commander can make changes as he/she sees fit such as civilian t shirt with ACU OCP bottoms.

The Commander can modify uniform requirements and wear within their own sphere o influence at any time. Technically, unit PT Shirts aren't authorized by AR 670-1 (or at least they didn't used to be). But that didn't mean each unit didn't have them. But when you go OUTSIDE that unit, good luck. I couldn't wear a DIFFERENT unit's PT Shirt. And I couldn't wear ANY unit PT shirt to an NCO Academy. This is really no different - Commander authorizing a different uniform for THAT UNIT only for that single event.

As far as wear in the civilian world, OCP / ACU / BDU pants are EXCEPTIONALLY good cargo pants. And they are widely available through army/navy surplus AND second-hand stores as servicemembers retire or they just change out their "cook whites" for new uniforms as part of their uniform maintenance. OCP / ACU / BDU pants are probably the single most common uniform item to find in the civilian world - popular with hunters, landscapers, survivalists, hikers, and even high schoolers. Personally, I would argue that their proliferance in the civilian world would take them OUT of the "exclusively military in character" realm and into simply camouflaged cargo pants.

You understand that Army Regulations only apply to people in the Army, right? Civilians can wear ACU bottoms with Dress Blue Tops while wearing Campaign Covers and ballet slippers if they feel like it.

Something to consider before this is why does it matter to you that Soldiers are using a t-shirt as part of a ruck or hike? Who is it hurting? Who is it offending? What is your purpose for this information?

When it's a recreational, optional, or similar event I have seen the uniform standard be relaxed for those events. I have also seen Soldiers switch to a t-shirt or hoodie for when they need to grab gas or something from the grocery store before heading home.

1LT (Join to see) has a good point. Unit t-shirts, hoodies, and other items are a very common thing. I myself have assisted multiple units in designing their own shirts to be worn with authorization from the unit commander and CSM or 1SG as applicable. This apparel is worn during the duty day, at PT, or at recreational events. It's not just company's and detachments that have unit apparel, you'll find battalions thru corps with their own unit apparel or authorized civilian clothes that are worn with the uniform for specific purposes or for esprit de corps.

So you being listed as a Civilian, the question must be asked. Why do you care? What's your intention with this information? I ask because I don't feel as though we have been given the full scope here.

Were they OCP or MultiCam?

He is a LTC he sets the Uniform of the day, he is probably a Battalion Commander so his word is law unless it is shot down by a higher rank

There are plenty of commercial purveyors of OCP patterned clothing. I'd say that prevents the pants from meeting that "exclusively military in character" requirement.
If it were a part of the uniform that included the name, rank, and service branch, then I could see it being a problem.