Where does the oxygen go? Drawing chemistry?

that takes PRACTICE practice practice...

when I (with all my __ years of experience in chemistry - nope.. I'm not filling in that blank!) read CH3OCH3 and CH3COCH3, I can glance at those formulas and know their structure. If the formulas are more complicated, it might take a bit more work. Point is, over time, it will get easier. The more you practice, the better you'll get!

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for now.. here's what you should think about and what you should do
.. (1) know the functional groups.. aldehyde, ketone, alcohol, carboxylic acid, ester, ether, etc.
.. (2) read from left to right. think about the carbons, each one must have 4 bonds.
.. (3) read things directly after a C as bonded to that C.. (that's usually the case)

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like this
.. CH3OCH3
from the left
.. CH3 is the methyl group.. it has 1 C with 3 bonds. needs 1 more bond
.. O needs 2 bonds. one is going to be bonded to the left C.. so 1 must be bonded to the right C
.. CH3 on the right is another methyl group
... .. .. .. H..... . .H
... ... . ...|.. .... .. |
.. ... H - C - O - C - H
... ... . ...|.. .... .. |
... .. .. .. H..... . .H
.. and we can write that
.. ... CH3 - O - CH3
.. if we wish... anyway, that is an ETHER... that is dimethylether... di-methyl-ether
.. it has an ether functional group.. .R - O - R' is ether.

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and the other compound
.. CH3COCH3

that also had the methyl group on the left, and on the right, but it has that extra C..

so let's read from left to right

.. . CH3 - C ___ - CH3

we know the O must be bonded 2x.
we know the central C must be bonded 4x
we know the right C must be attached to something

this is the only way that fits.
.. ... ... ... .O
.. ... ... .. ..||
.. . CH3 - C - CH3

which is our ketone functional groups.. dimethylketone

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some other examples
.. CH3CHOHCH2CH3

left to right
.. CH3 - C - ___ CH2CH3
that's pretty obvious right?

not the H and OH after that 2nd C... they are both attached to the C
.. ... ... ..H
... ... ... ..|
.. CH3 - C - CH2CH3
.. ... ... .. |
.. ... ... ...OH

this is an alcohol functional group... R - OH... 2-butanol

and this example
.. CH3CO2CH3

is an ester..
.. .... ... .O
.. ... .. ... ||
.. CH3 - C - O - CH3.... .. . ester functional group... methylacetate.

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again.. think about (1), (2) and (3) above and PRACTICE

For CH₃OCH₃, 2 CH₃- groups are attached to an oxygen atom, which forms 2 covalent bonds to the 2 CH₃- groups. Hence, it would be
CH₃-O-CH₃.

For CH₃COCH₃, 2 CH₃- groups are attached to a -CO- group. The -CO- group should be a C=O group instead of -C-O- group (this is impossible as the carbon atom forms only 2 covalent bond). Hence, it should be
CH₃-CO-CH₃.

The structural formulae of the two compounds are shown below.