Assuming the coin is circulated the 1921-S Morgan is so common retail values are only $16.00-$21.00
Morgan dollars were minted from 1878 to 1904 and in 1921. Please check the back of your coin. You have a Peace dollar rather than a Morgan.
The last Morgan dollars were dated 1921. Your coin would be a PEACE dollar. Please see the Related Question for details.
The 1921 Morgan regardless of mintmark is the most common of all Morgan dollars. An circulated coin is $37.00-$44.00
For the Morgan dollars, 1921 was the last year of issue and the highest mintage. They were struck at all 3 Mints. Current retail value of a 1921-S is $30.00-$40.00 for circulated examples depending on the grade. Uncirculated are $45.00-$55.00 for most coins.
4-28-11>> Assuming the coin is circulated the 1921-S Morgan is so common retail values are only $37.00-$44.00
Peace dollars were made from 1921 to 1935. An 1879 silver dollar is called a Morgan dollar after its designer George T. Morgan. There's more information at the Related Question
The is no 1921S Peace dollar; Peace dollars were not produced in San Francisco ("S" mintmark) in 1921. If your 1921 dollar coin is a Peace dollar, it was produced in Philadelphia and has no mintmark; if it has an "S" mintmark, it is a Morgan dollar produced in San Francisco).
Please check your coin again. 1921 Peace dollars were only struck at Philadelphia. If the eagle on the back of your coin has outstretched wings it's a Morgan dollar rather than a Peace dollar; both types were struck that year. There's more information at the Related Question. Check that coin again. The only Peace dollars dated 1921 were minted in Philadelphia. However, a 1921-S Morgan is currently (2014) worth around $25.
7-28-11>>> Of all the Morgan dollars, the 1921 dated coins are the highest mintage most common. Regardless of mintmarks, values for circulated coins are $36.00-$44.00.
I am not aware of a 1921 Morgan Dollar that carries a mint mark of "O." However, the 1921 series do carry mint marks of "D'" and "S." However, what is interseting about the 1921 Morgan Dollars is that they were produced by the US mint after a hiatus of 17 years - secondary to the The Pittman Act that required that all silver dollars be melted. More info - There could not be a 1921-O dollar because the New Orleans mint closed in 1909. 1921 was the only year that Morgan dollars were struck in Denver, because the Denver mint opened in 1906, two years into that hiatus.
If you know it's a 1921 "S" Morgan you found the mint mark
Without knowing their dates it's not possible to give a value. Morgan dollars were minted from 1878 to 1904 and again in 1921. Please post new questions with that information, one for each coin if they have different dates.