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documentclass[11pt, oneside]{article}

usepackage{geometry}

geometry{a4paper}

usepackage{graphicx}

usepackage{amssymb}

usepackage{arydshln}

usepackage[table]{xcolor}

usepackage{pifont}



begin{document}



begin{center} renewcommand*arraystretch{1.2}



scalebox{1}[1]{|c|{begin{tabular}[t]{|rrl||c|c|c|c||c|} hlinemulticolumn{3}|} & {sc{c1}} & {sc{c2}} & {sc{c3}} & {sc{c4}} & {textit{H}} \[0.5ex]



& & & {textit{w=7}} & {textit{w=3}} & {textit{w=3}} & {textit{w=1}} & \



hline hline a. & ding{43} & {O1} & & -3 & & -1 & -4 \



hline b. & & {O2} & & -3 & -3 & -1 & -7 \



hline c. & ding{43} & {O3} & & & -3 & -1 & -4 \



hline end{tabular} renewcommand*arraystretch{1} end{center}



end{document}

I suggest to simplify your syntax quite a bit:

• instead of the center environment, which adds additional vertical spaces, you could use centering




• the scalebox{1}{1}{...} does not have any effect (scales both directions by 1), thus can be left out




• your second row has not enough entries, I suspect one empty cell is missing at the start




• two-letter font commands are deprecated, see Does it matter if I use textit or it, bfseries or bf, etc (and just my personal opinion, the combination between small caps and numbers looks odd...)




• for better spacing put your equations w=3 in math mode instead of italics, same for all the negative numbers. This will give you real minus signs instead of hyphens




• many of the {} around the cells are unnecessary (thank to @Au101 for pointing this out in the comments)




• and what Werner and Simon say in their comments below your question

and please have a look at http://betterposters.blogspot.com/2012/08/the-data-prison.html

documentclass[11pt, oneside]{article}

usepackage{geometry}

geometry{a4paper}

usepackage{graphicx}

usepackage{amssymb}

usepackage{arydshln}

usepackage[table]{xcolor}

usepackage{pifont}



begin{document}



begingroup

centering

renewcommand*arraystretch{1.2}

begin{tabular}[t]{|rrl||c|c|c|c||c|} 

    hline

    multicolumn{3}{|l}{} & textsc{c1} & textsc{c2} & textsc{c3} & textsc{c4} & textit{H} \[0.5ex]

    & & & & $w=7$ & $w=3$ & $w=3$ & $w=1$ &\

    hline hline 

    a. & ding{43} & O1 & & $-3$ & & $-1$ & $-4$ \

    hline 

    b. & & O2 & & $-3$ & $-3$ & $-1$ & $-7$ \

    hline 

    c. & ding{43} & O3 & & & $-3$ & $-1$ & $-4$ \

    hline 

end{tabular}



endgroup





end{document}

Maybe this is what you want?

documentclass[11pt, oneside]{article}

usepackage{geometry}

geometry{a4paper}

usepackage{graphicx}

usepackage{amsmath, amssymb}

usepackage{hhline}

usepackage{arydshln}

usepackage[table]{xcolor}

usepackage{pifont}



begin{document}



[ renewcommand*arraystretch{1.2}

begin{array}

{|rrl||c|c|c|c||c|}

 hhline{---||----||-}

textsc{c1} & & & {textsc{c2}} & {textsc{c3}} & {textsc{c4}} & {H} & \[0.5ex]

text{&} & & {w=7} & {w=3} & {w=3} & {w=1} & & \

hhline{===::====::=}

a. & text{ding{43}} & {O1} & & -3 & & -1 & -4 \

hhline{---||----||-} b. & & {O2} & & -3 & -3 & -1 & -7 \

hhline{---||----||-}

 c. & text{ding{43}} & {O3} & & & -3 & -1 & -4 \

hhline{---||----||-}

 end{array}]%



end{document}