The players mentioned in Part 3 are those I would expect other teams may call about, but aren't players EDM really needs to actively shop in an effort to clear cap room at the moment. I think one could make a reasonable argument that O'Sullivan fits in this group. I think it's more clear that these players are either more worth their money, or are younger and therefore have a bit more potential for the Oilers to extract value from going forward.
Penner and Visnovsky are players that Edmonton should hold onto for next season if they plan on competing for a playoff spot. They are positive difference makers producing, at the least, a reasonable contribution to the team given their salary/cap hit. I don't think it's clear that either could be replaced, bang for buck, on the UFA market next summer, and the Oilers clearly don't have an internal replacements at the ready. That said, they are close enough to UFA, and have enough potential warts that if the price were right, moving them at the deadline might be advantageous long term, even if it hurts the team next season.
These 4 players may or may not be considered core players by the Oilers brass, but I'm not sure any of them are likely to carry higher value now than they will in the summer. I wouldn't be surprised if the Oilers received calls about all these players somewhat regularly, and one wouldn't be terribly surpried to find they are considered part of the core. Even if they aren't, you don't ordinarily see need-based trades made between clubs at this point in the season. If the Oilers are planning to move Cogliano for a D, or a bigger forward, or as part of a package, it would seem more likely to be consummated in the summer than now, IMO.
I'm tempted to group these players with the 4 forwards above; I'm probably oversorting by separating these two defencemen. I would guess it's a bit more likely that one of them/they move vs. the 4 players above IF the team acquiring them is looking to add a D for a playoff run. Generally veterans, both at F and D, are more likely to move as deadline deals, but these two D are a bit older and more seasoned than those listed above, and may hold more appeal to a team both looking at bolstering this year's D for a cup run AND their D going forward.