I am not sure if OH has a flow-thru agreement and doubt if DL would state that they are giving preferential treatment but OH pilots are probably higher than average seniority because the company is older than most regional carriers and was an early RJ operator.... I'm sure many pilots do regularly move on to the majors but the combination of OH's age and RJ experience plus OH's operations in the congested NE probably make OH pilots reasonably attractive on their own.... yes, I know that is a generalization and there are other carriers that fit that description as well...
My guess is that the OH capacity cuts were already factored into the DL capacity guidance... DL has been pretty aggressive in stating they intended to get 50 seaters out of the system. If you look at most of the major airlines' traffic reports, the regional carriers operate about 12-15% of the total system capacity right now.... given that DL has hundreds of regional jets in its operation (flying under contract), removing 50 of the smallest jets will probably have a fairly small impact. When you factor in that DL is adding seats to the MD80 and MD90 fleets and buying used MD90s to replace D9s, there probably isn't a big change in capacity overall.
It does say that the trend to larger aircraft is accelerating which both reduces CASM - allowing DL to compete more effectively with low cost carriers - and reduces frequencies which should help w/ ATC congestion.
At the same time, there really is not a need for as many smaller regional jets as the industry consolidates. Cities in the middle of the country that have had service to 2-3 hubs for each carrier mostly on RJs don't need that level of service when you combine carriers... such as DL/NW did and UA/CO will do. Small to medium sized cities like GRR, OKC, BNA etc that have largely seen a shift to RJs over the past decade will now see a shift back to more mainline jets although the number of flights will be reduced. As with many things, the trend is to fewer large hubs using larger aircraft than to have lots of smaller hubs that needed alot of RJs in order to make those hubs work.
Overall, the winding down of the age of 50 seaters is good for mainline airline employees and won't be missed by too many passengers... with enough notice and planning and growth at the major and low fare carriers, regional carrier employees might successfully transition but the poor health of the overall economy will make it a challenge for some....