Interracial breeding is possible in Elder Scrolls. Bretons are the most obvious example of this (even though Imperials have elven blood too). However, as you probably know, character creation does not support mixed breeds. And an in-game book (I believe it is Notes on Racial Phylogeny) explains that interracial breeding occurs, but it also mentions, specifically with an Orc, that a Bosmer woman would be rather ashamed or embarrased at revealing that she has a child with an Orc father.

Of course, that there confirms that Orcs can have children with the other human races. Still, one problem remains: the actual character creation for your half-orc. Lore is generally lacking in the area if men and mer (as you know, Orcs are a type of mer) are the same species, or if they are different species. That right there is the biggest thing to consider, this reason:

If men and mer are part of the same species, we can draw this example to the 3 major 'races' (Negroid, Mongoloid, and Aryan I believe) and countless minority races (Dravidian, Ainu, Polynesian) (mind you that the human races have the potential to not even be considered races due to their genetic similarity) and when they have children together. A mixed child for us is an intermediary form of both races. A black and white person's child will have neither skin tone, instead a mix. A mix of all features. Children generally become fertile (from medical problems unrelated to mixed racial background). And, judging from the fact that Bretons exist as a reproducing race, it would seem as if men and mer are the same species. That would mean that your half-Orc would be best fit as a mix between two races, where you could modify some features, namely facial features, that would come from the mother race, and the father race.

However, the other kind of mix, where members are not of the same species, are different. We see this mostly in animals, such as the mule, or ligers. However, whether or not you wish to take it is truth, it is believed that modern *** Sapiens (us) have Neanderthal DNA in us, and Neaderthals, according to our current knowledge, were a different species. Well, as you have probably found out, will find out, or probably already know, humans are fertile. However, many cross-species animal hybrids are infertile for several reasons. Now what did that have to do with anything? Well, I thought I had a good idea, but then I realized that animal hybrids are mixes of their parents. However, something I could point out to argue that men and mer are different species, barring the fact that their mixed offspring are fertile (which could just be how modern humans are with supposed small amounts of Neanderthal ancestry), is the vast difference in physical traits that men and mer have. Men are generally thicker and wider, alongside having a notable different jaw shape than mer, who tend to be thinner. Then brings in eye color, which mer and men have several differences in, both in terms of overall eye shape and structure, and varieties of color. Today, brown eye color is shared between all modern humans, and generally European peoples are the only have more than two types of eye color (Other Aryan peoples tend to have a few non-brown eye colors, such as green). And then, what is most visible in Elder Scrolls races: magical potential. Altmer and Dunmer, in lore, have more magical potential than Nords or Redguards. While IRL humans do not have magical potential, it would be safe to say that all human races, if we have magical potential, would be about equal over all races due to genetic similarly, if magical potential even is genetic. Nevertheless, a half-Orc would probably still look like a mix of his parents.

Of course, some of the people on the forums say that a mixed-blood in Elder Scrolls lore looks like the mother. Due to a significant lack of mixed-race people in game, it is rather hard to say for certain if that is indeed true, or only partially true.

So, in conclusion, you could either make your half-Orc look like the race of one of their parents (Orc or whatever the other parent is), or you could make them look like a mix of both, however, that may be a bit harder, but still possible if you are determined enough, I'm sure.

Author's note: Khajiit and Saxaheel/Argonians were not included because it would seem as if they are indeed different species from men and mer.